docs: add TESTING.md for python

[opieter-aws]

Description

The TypeScript SDK has a thorough docs/TESTING.md that strands-ts/AGENTS.md hard-delegates to ("you MUST follow"). The Python SDK had no equivalent — its testing guidance was ~40 lines inside AGENTS.md and nothing in docs/. The practical cost is that real, reusable test infrastructure stays invisible: the shared fixtures (MockedModelProvider, MockHookProvider, MockAgentTool, MockedSessionRepository) and the conftest.py async/AWS helpers (agenerator, alist, generate, moto_*) aren't documented anywhere, so contributors hand-roll their own mocks instead of reusing them. A few sharp edges also went unwritten — strict-asyncio mode silently skips an unmarked coroutine test, and asserting a whole Message dict when a test only cares about role/text is a recurring source of brittle breakage in tests_integ/.

This adds strands-py/docs/TESTING.md as the authoritative Python testing reference, mirroring the TypeScript guide's depth and topic coverage so the two SDKs stay aligned. It documents the fixtures quick-reference (with "reuse these, don't hand-roll"), the tru_/exp_ assertion-pair naming, whole-object assertions with unittest.mock.ANY for volatile fields, the strict-asyncio marker requirement, and the flat-vs-class test-organization preference — all reflecting conventions the suite already follows rather than inventing new ones.

It also corrects a contradiction in strands-ts/docs/TESTING.md, which referenced a tests_integ/ directory that doesn't exist; the integration tests live in test/integ/ (as the same file states a few lines later). An agent following the authoritative doc would otherwise create files in the wrong place.

The companion AGENTS.md restructure that links to this new file is a separate PR; this one is split out so the doc it points at lands first.

Related Issues

None — maintenance change surfaced by a convention audit of both SDKs.

Documentation PR

Not applicable — these are contributor-facing docs/ files in the SDK packages, not user-facing documentation under site/.

Type of Change

Documentation update

Testing

Documentation-only. Verified that every fixture, helper, and path named in the new guide exists in the tree (tests/fixtures/*, the conftest.py session-scoped helpers, strict-asyncio being the default), and that the corrected test/integ/ path matches the real TypeScript integration-test directory.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Assessment: Approve

Documentation-only change that adds an authoritative Python TESTING.md mirroring the TypeScript guide, plus a one-line path correction in the TS doc. I verified every factual claim against the tree and they all hold up.

Verification details

• Layout & paths: tests_integ/ and the src/strands/ ↔ tests/strands/ mirror both exist as described.
• Fixtures table: all five classes (MockedModelProvider, MockHookProvider, MockMultiAgentHookProvider, MockAgentTool, MockedSessionRepository) exist at the listed paths with the stated names; MockedModelProvider does accept per-response Usage.
• conftest helpers: agenerator, alist, generate, moto_env, moto_mock_aws all present; generate returns (events, return_value) as documented.
• Strict asyncio: asyncio_mode is indeed unset in pyproject.toml, so the strict default holds.
• Python 3.10–3.14: matches requires-python, classifiers, and the hatch matrix.
• TS fix: test/integ/ is the real directory (the old tests_integ/ reference was stale), consistent with strands-ts/AGENTS.md.
• Conventions: tru_/exp_, whole-object assertions, and unittest.mock.ANY masking are all genuinely used in the suite — the doc reflects existing practice rather than inventing it.

Only one non-blocking clarity nit inline (the two assertion rules can read as contradictory). Clear, accurate, and tightly scoped — nice work splitting the AGENTS.md restructure into a separate PR.

[github-actions]

Assessment: Approve

Re-reviewed after the "Clarify wording" commit (2d7a743). The Assertions section now opens with "The right granularity depends on who controls the expected shape" and recasts the two rules as a primary rule plus a natural exception — which fully resolves the only point from my earlier review. The two directives no longer read as contradictory.

No other changes since the last pass; the TS path correction is unchanged and still accurate. Nothing outstanding — good to merge.

[github-actions]

Assessment: Approve

Re-reviewed after the "Address comments" commit (95ef186), which responds to all three of @pgrayy's review comments:

How each comment was addressed

• Drop the TS cross-reference / keep this Python-specific → The intro paragraph no longer mentions the TypeScript SDK; it now reads as a standalone Python reference. ✅
• Test naming convention → A new bullet codifies test_<method>_<description> (with the test_<subject>_<method>_<description> variant for ambiguous modules) and the class-scope exception. I fact-checked both cited examples — test__init__default_model_id (in test_bedrock.py) and test_update_config_validation_warns_on_unknown_keys (across the model tests) — and they exist verbatim. The "most of the suite is flat" claim also holds: ~1856 flat test functions vs ~246 Test* classes. ✅
• Centralize test rules / link from AGENTS.md → Author confirmed this is the follow-up PR docs: improve agent guidance #2959, deliberately split to keep this one small. ✅

One thing worth confirming with the reviewer: this Python doc intentionally keeps tests_integ/ (the real Python integ path), while the TS-doc fix in this same PR changes the TS path to test/integ/. Both are correct for their respective SDKs — just flagging since the two paths now differ across the two files.

Nothing blocking. Accurate, tightly scoped, and all reviewer feedback addressed.