fix(wrap): preserve pre-set ANTHROPIC_BASE_URL as proxy upstream

[ousamabenyounes]

Description

Issue #1353: when the user already has ANTHROPIC_BASE_URL pointing at a LiteLLM (or any custom Anthropic-compatible) gateway, headroom wrap claude overwrote it with the local proxy URL and silently forwarded to api.anthropic.com. wrap claude now treats a pre-existing non-self-pointing ANTHROPIC_BASE_URL as the proxy's upstream (passed as ANTHROPIC_TARGET_API_URL) so compression layers on top of the user's gateway instead of replacing it. Explicit ANTHROPIC_TARGET_API_URL, Foundry and Vertex paths still win.

Closes #1353

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Add _detect_inbound_anthropic_upstream(proxy_port) to headroom/cli/wrap.py: returns a pre-set ANTHROPIC_BASE_URL as the proxy upstream, ignoring unset/blank values and self-pointing localhost URLs on the proxy port.
• Wire it into wrap claude so a user's custom gateway is preserved as ANTHROPIC_TARGET_API_URL instead of being overwritten.
• Precedence unchanged: explicit ANTHROPIC_TARGET_API_URL, Foundry, and Vertex still take priority.
• Add tests/test_cli/test_wrap_claude_inbound_upstream.py (8 cases) covering detection, blank/unset, self-pointing URLs, and a different-port upstream.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

tests/test_cli/test_wrap_claude_inbound_upstream.py::test_detects_custom_litellm_upstream PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_when_unset PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_when_blank PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_for_self_pointing_url[http://127.0.0.1:8787] PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_for_self_pointing_url[http://localhost:8787] PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_for_self_pointing_url[http://127.0.0.1:8787/] PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_none_for_self_pointing_url[http://LOCALHOST:8787/v1] PASSED
tests/test_cli/test_wrap_claude_inbound_upstream.py::test_returns_localhost_on_different_port PASSED

============================== 8 passed in 0.25s ===============================

Real Behavior Proof

• Environment: Python 3.13, local checkout, pytest 9.0.3
• Exact command / steps: pytest tests/test_cli/test_wrap_claude_inbound_upstream.py -v on the patched branch, then with the production fix reverted.
• Observed result (RED → GREEN):
  • RED — with the fix reverted, all 8 tests fail: AttributeError: module 'headroom.cli.wrap' has no attribute '_detect_inbound_anthropic_upstream' (8 failed in 0.37s).
  • GREEN — with the fix applied, all 8 pass (8 passed in 0.25s).
• Not tested: end-to-end live LiteLLM gateway round-trip (covered indirectly by the upstream-URL detection unit tests).

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Additional Notes

Docs/CHANGELOG checklist items left unchecked as N/A: this is a behavioral bug fix to existing wrap claude upstream handling with no new public surface to document.

Real Behavior Proof

• Environment: headroom 0.28.0 dev checkout (branch fix/issue-1353), .venv, proxy port 8787.
• Exact command / steps: invoked the real detection path used by wrap claude (_detect_inbound_anthropic_upstream) across representative ANTHROPIC_BASE_URL values:

  python -c "from headroom.cli.wrap import _detect_inbound_anthropic_upstream as d; ..."
  

• Observed result:

  custom LiteLLM gateway        -> 'https://litellm.mycorp.internal/v1'
  unset                         -> None
  blank                         -> None
  self-pointing localhost:8787  -> None
  self-pointing LOCALHOST/v1    -> None
  different port (real upstream)-> 'http://10.0.0.5:4000'
  

  A pre-set, non-self-pointing ANTHROPIC_BASE_URL is returned as the proxy upstream (wired to ANTHROPIC_TARGET_API_URL), so compression layers on top of the user's gateway. Unset / blank / self-pointing values fall through to None, so the proxy default applies and no self-forwarding loop is created.
• Not tested: full wrap claude launch end-to-end against a live LiteLLM gateway (needs the external gateway + Claude Code); the wiring from custom_upstream to ANTHROPIC_TARGET_API_URL is covered by the unit suite.

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[JerrettDavis]

Reviewed the wrap claude inbound upstream handling and ran uv run --with pytest python -m pytest tests/test_cli/test_wrap_claude_inbound_upstream.py -q: 8 passed. The precedence with explicit target/Foundry/Vertex is preserved, and self-pointing proxy URLs are ignored to avoid loops. No blocking issues found.