fix: extend retry predicate to cover transient server errors and connection failures

[VANDRANKI]

What does this PR do?

Closes #2165

ApiModel already has retry logic via Retrying, but the predicate is_rate_limit_error only matches 429 / rate-limit signals. Transient server errors (502, 503, 504, 500) and connection failures (reset, timeout) are not retried today — the agent run fails immediately even though a retry would succeed.

This PR adds is_transient_error, a broader predicate that covers:

• Rate limits (429, "rate limit", "too many requests")
• Server-side transient errors (500, 502, 503, 504)
• Connection-level failures (reset, refused, timeout)

Non-retryable errors (400 bad request, 401 unauthorized, 404 not found) are not matched and still fail immediately.

ApiModel now uses is_transient_error as its retry predicate. The old is_rate_limit_error is kept for backwards compatibility.

Tests

Added TestIsTransientError in tests/test_models.py with 11 cases: rate limit variants, each server error code, connection reset, timeout, and three non-retryable codes that must not match.