fix(agent): default context lookup for empty model IDs

[whirmill]

Summary

This is the focused current-main replacement for the unique safety fix in #26873.

get_model_context_length() can be called while a runtime/model selection is still empty. Before this guard, an empty or None model flowed into provider-prefix normalization and then into cache/probe logic. Return the documented 256K fallback immediately instead.

The replacement intentionally excludes the unrelated runtime-switch, Feishu, cron, auxiliary-client, metadata, and compressor changes from #26873. Those concerns overlap with later upstream work and should not be resurrected as one conflicted patch.

Verification

• scripts/run_tests.sh tests/agent/test_model_metadata.py -q — 122 passed
• ruff check agent/model_metadata.py tests/agent/test_model_metadata.py
• python -m py_compile agent/model_metadata.py tests/agent/test_model_metadata.py
• git diff --check

Closes the unique empty-model failure mode extracted from #26873; the historical PR remains separate until maintainers decide how to handle its now-overlapping runtime-switch changes.

[teknium1]

Thanks for extracting the focused empty-model case from #26873. The premise still holds on current main: agent/model_metadata.py:2166 passes model to _strip_provider_prefix, whose string-membership operation at agent/model_metadata.py:95 raises for None; an empty string can continue to metadata resolution at agent/model_metadata.py:2460.

Problems

• The patch needs a small manual relocation: current main inserted malformed-URL handling at agent/model_metadata.py:2152-2161, so the submitted hunk does not apply. Place the guard after the custom-provider override and before that URL handling.
• tests/agent/test_model_metadata.py should prove the immediate-return contract. The empty-string assertion alone could pass after the current resolver performs the OpenRouter metadata lookup at agent/model_metadata.py:2461-2472 and eventually falls back to 256K.

Suggested changes

• Add whitespace coverage and mock/assert that blank input does not enter a downstream lookup/probe path.

Automated hermes-sweeper review.

[teknium1]

This verifies the final value but not the stated early-return guarantee: on current main, "" can reach fetch_model_metadata() and still eventually return the same fallback. Please assert that a downstream resolver/probe is not called (and include whitespace input).

[kshitijk4poor]

Salvaged in #85498 (rebased onto current main with your commit and authorship preserved). Your fix turned out to be load-bearing: the empty-model fuzzy-match footgun you guarded against started failing CI on main for every open PR this week (test_allowed_for_nous_anthropic_messages — the "" model matches an arbitrary entry from the live Nous portal catalog, which now yields a 32K context and trips the 64K floor). Thanks for the fix!