fix(agent): default context lookup for empty model IDs (salvage #65515) — un-reds main CI

[kshitijk4poor]

Summary

Every open PR's CI is currently red on one shard with:

FAILED tests/run_agent/test_primary_runtime_restore.py::TestTryRecoverPrimaryTransport::test_allowed_for_nous_anthropic_messages
ValueError: Model  has a context window of 32,000 tokens, which is below the minimum 64,000 required by Hermes Agent.

(Seen on #85444 slice 7/12, #85452 slice 2/12; reproduces locally on plain upstream/main files.)

Root cause: the test constructs an agent with model="" against the live Nous portal URL. get_model_context_length("") reaches _resolve_endpoint_context_length, whose fuzzy matcher

if model in key or key in model:

is vacuously true for an empty model — "" in key matches every entry — so it picks an arbitrary model from the live /v1/models response and returns that entry's context length. The portal catalog changed recently; the arbitrary match now lands on a 32K entry, init_agent raises the 64K-floor ValueError, and the junk value is persisted under a @https://inference-api.nousresearch.com/v1 cache key (visible in the failing log: Cached context length @https://... -> 32,000 tokens).

This is a salvage of #65515 by @whirmill (open since July 16), rebased onto current main — the same guard, now positioned after the malformed-base_url normalization that landed since, plus an explanatory comment documenting the fuzzy-match footgun. Contributor authorship preserved on the commit.

Changes

• agent/model_metadata.py: a blank/empty model id falls back to DEFAULT_FALLBACK_CONTEXT immediately — before any cache write or network probe.
• tests/agent/test_model_metadata.py: test_empty_model_uses_fallback_context (covers "" and None).

Validation

• The two previously failing/new tests pass: test_allowed_for_nous_anthropic_messages + test_empty_model_uses_fallback_context.
• Full tests/agent/test_model_metadata.py + tests/run_agent/test_primary_runtime_restore.py: 92 passed.
• Mutation check: removing the guard makes test_empty_model_uses_fallback_context fail (arbitrary live-catalog match returns non-default), restoring goes green.
• ruff check clean.

Closes #65515.