fix(discovery): bound tool-call probe response reads (#1041)

[seonghobae]

Summary

Closes the last unbounded provider-body read in model discovery. probe_discovered_model_tool_call_capability read both its success body and its HTTP 400 error body with a bare response.read(), while every sibling fetch (_fetch_json, _fetch_json_same_host_https, _fetch_configured_gateway_json) caps its read at MAX_DISCOVERY_RESPONSE_BYTES. A compromised or misbehaving provider could therefore stream an arbitrarily large body into memory during a capability probe.

Changes

• contextual_orchestrator/model_discovery.py
  • Success body read is capped at MAX_DISCOVERY_RESPONSE_BYTES + 1; an oversized body is treated as ambiguous evidence and returns None (fail-closed), never buffered whole.
  • HTTP 400 error body read uses the same cap; an oversized error body returns None.
  • Docstring documents the untrusted-input bound.
• tests/test_model_discovery_boundaries.py
  • RED-first: caps success-body read, rejects oversized success body without buffering, caps 400 rejection body read, and preserves the in-budget explicit single-call rejection (False).
  • Focused suite: 25 passed.

Verification

• pytest tests/test_model_discovery_boundaries.py -q -> 25 passed
• pytest tests/test_model_discovery.py tests/test_model_discovery_boundaries.py tests/test_discover_models_cli.py -q -> 169 passed
• Full suite: pytest -q -> 3497 passed, 2 skipped
• python -m compileall -q contextual_orchestrator tests -> clean; git diff --check -> clean

Scope

Independent slice under #1041, branched from origin/main (abc2dea8) in its own worktree. Does not touch the PR #1121 review head. ModelClient default transport and EgressWeave adoption remain a separate design item.

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