Migrate anthropic provider to anthropic SDK 1.0 (httpx2) and remove the <1.0.0 cap

[o-nikolas]

Workaround being applied

The anthropic provider is getting a temporary upper bound anthropic>=0.121.0,<1.0.0 (main dependency and the bedrock/vertex/aws extras).

Why

anthropic 1.0.0 (released 2026-08-20) replaces httpx with httpx2 (pydantic's fork). Once the automated CI environment upgrade bumped uv.lock to anthropic==1.0.0 (#72016, commit b5ab83f), "MyPy providers checks" started failing on every PR built on top of it:

providers/anthropic/tests/unit/anthropic/hooks/test_anthropic.py:287: error:
Argument "response" to "BadRequestError" has incompatible type
"httpx._models.Response"; expected "httpx2._models.Response"

This blocks unrelated PRs (first seen on the provider release PR #72069). Beyond the type error, 1.0 is a major SDK release and the provider has not been validated against it, so a cap is safer than a spot-fix of the test.

Follow-up work

• Review the anthropic SDK 1.0 changelog / breaking changes and validate the provider (hooks, operators, triggers, sensors) against it.
• Switch the test helpers that construct httpx.Request/httpx.Response to httpx2 equivalents.
• Bump the minimum to a 1.x floor if the migration requires it, or keep dual compatibility if feasible.
• Remove the <1.0.0 cap from providers/anthropic/pyproject.toml (main dependency and extras) and regenerate uv.lock.

Acceptance criteria for removing the cap

• uv.lock resolves anthropic to a 1.x version.
• MyPy providers checks and the anthropic provider unit tests pass against anthropic 1.x.

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Drafted-by: Kiro CLI (claude-fable-5) (no human review before posting)