feat(provider): add Anthropic-specific context overflow markers

[bukeeastrey]

Summary

Anthropic's API returns context overflow errors using distinct phrasing and error types compared to OpenAI-compatible providers. Without matching markers in _is_context_overflow(), Anthropic overflow errors fell through to ProviderFailureKind.BAD_REQUEST (via invalid_request_error), bypassing the COMPACT_AND_RETRY recovery action and immediately failing out to the user.

This PR adds dedicated markers to _is_context_overflow() to properly classify Anthropic context overflows:

• "prompt_too_long" — matches raw error code / proxy responses
• "exceed context limit" — matches Anthropic's canonical 400 error message ("input length and max_tokens exceed context limit: ...")
• "request_too_large" — matches Anthropic's 413 error type
• "request size exceeds" — matches the 413 payload message variant

(Note: "max_tokens" was intentionally omitted as a standalone marker to avoid false positives on valid requests mentioning parameter names).

Changes

• src/agentos/provider/failures.py: Added Anthropic-specific markers to _is_context_overflow().
• tests/test_provider_failure_classification.py: Added unit tests covering canonical Anthropic 400 messages, 413 error types, and negative cases ensuring non-overflow invalid_request_error stays BAD_REQUEST.

Verification

• uv run ruff check src/agentos/provider/failures.py tests/test_provider_failure_classification.py (Passed)
• uv run mypy src/agentos/provider/failures.py --show-error-codes (Passed)
• uv run pytest tests/test_provider_failure_classification.py -q (93/93 tests passed)
  closes [Feature]: Add Anthropic-specific markers to _is_context_overflow for context overflow classification #613

[andreapn]

Issue #613 has been resolved by #615, which landed on main with the same four Anthropic markers (prompt_too_long, exceed context limit, request_too_large, request size exceeds).

Closing this as superseded. Thanks for the contribution — the negative test in this PR (invalid_request_error without overflow markers stays BAD_REQUEST) is a nice addition and would be welcome as a small follow-up PR against current main if you'd like to open one.