fix(bedrock): resolve context length via static table before custom-endpoint probe

[AndreKurait]

What does this PR do?

Fixes a resolution-order bug in get_model_context_length() that caused every AWS Bedrock model to report the 128K default-fallback context length instead of the correct value from the static Bedrock table (200K for Claude, etc.).

The predicate for Bedrock detection is unchanged — only the position of that branch in the resolution order moves from step 4b to step 1b, so it runs before the custom-endpoint probe.

Why?

bedrock-runtime.<region>.amazonaws.com is not in _URL_TO_PROVIDER, so _is_known_provider_base_url() returns False for Bedrock URLs. The original resolution order then:

1. Treated Bedrock as a custom endpoint (_is_custom_endpoint → True).
2. Called fetch_endpoint_model_metadata() → GET /models on the Bedrock URL (Bedrock doesn't serve this shape).
3. Matched no models → fell through to return DEFAULT_FALLBACK_CONTEXT (128K) at the "probe-down" branch on line 1109.
4. Never reached the Bedrock branch at step 4b.

Result: users on Bedrock saw 128K context for Claude Sonnet/Opus 4.x (which actually support 200K on Bedrock), triggering premature auto-compression and reducing usable context.

Changes

agent/model_metadata.py

• Promote the Bedrock static-table branch from step 4b → step 1b, so it runs before the custom-endpoint probe.
• Remove the now-dead step 4b block, replaced by a breadcrumb comment.
• Update the resolution-order docstring to reflect the new step 1b position.

tests/agent/test_model_metadata.py

• New TestBedrockContextResolution class (3 regression tests):
  • provider="bedrock" returns the static-table value and does not call fetch_endpoint_model_metadata (assert-not-called is the regression guard — the bug would have called it).
  • base_url="https://bedrock-runtime.us-west-2.amazonaws.com" works without an explicit provider hint.
  • Non-Bedrock custom endpoints still reach the probe (no over-reach).

Which type of PR is this?

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

pytest tests/agent/test_model_metadata.py -q
# 83 passed in 1.95s (3 new + 80 existing)

Also manually verified with provider="bedrock" on Claude Opus 4.7 — returns 200000 rather than 128000.

Risk

Very low:

• Predicate is identical to the original step 4b — no behaviour change for non-Bedrock paths.
• Original step 4b was dead code for the user-facing case (always hit the 128K fallback first), so removing it cannot regress behaviour.
• Bedrock path now short-circuits before any network I/O — faster too.
• ImportError fall-through preserved for users without boto3 installed.

Checklist

• My changes follow the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (docstring updated)
• My changes generate no new warnings
• I have added tests that prove my fix is effective

Related work

Complements #14710 (stale-connection client eviction on Bedrock) — correct context sizing is a prerequisite for the compression logic triggered by the error paths in that PR.

[teknium1]

Thanks for the fix, @AndreKurait! This has been merged to main.

Automated hermes-sweeper review.

• The Bedrock static-table branch (step 1b) is present in agent/model_metadata.py at the correct position, before the custom-endpoint probe — commit b290297d.
• TestBedrockContextResolution regression test class landed in tests/agent/test_model_metadata.py (line 595).
• The fix is on main but post-dates the v2026.4.23 release tag; it will ship in the next release.