fix(sdk): align error types, retry delays and response validation

[moonyue-w]

Summary

Align Python response handling with the pinned Anthropic v1.7.0 baseline. HTTP 413 and 529 receive dedicated error classes, positive server-requested retry delays above 60 seconds are respected, and responses use lenient model construction by default with optional strict validation.

• Export RequestTooLargeError and OverloadedError from qca and qca.common.
• Cap positive retry delays at 4,294,967 seconds and align millisecond-header precedence and backoff behavior. Retry eligibility is unchanged.
• Preserve _strict_response_validation=True across client copies, response views, pagination, and SSE.
• Add six read-only strict-response integration checks for Forward model/template/session lists and Managed model/agent/session lists. Existing business scenarios retain default parsing.

Why

Provide the same error classification and response compatibility behavior as Anthropic, honor server-requested retry waits, and retain a separate integration check for server schema drift.

Verification

• Ruff lint and format checks passed.
• mypy src/qca scripts: 595 source files passed.
• pytest -q: 2,994 passed; account-backed integration tests excluded.
• pytest tests/integration -m integration -k strict_response_contract -q: 6 passed against the real service using existing local test credentials; GET requests only.
• Documentation generation/check passed; 25 README snippets compiled.
• Wheel and source distribution built successfully.
• Example --help checks were not rerun; examples are unchanged.

Impact

• Public behavior changes and migration notes recorded in CHANGELOG.md and README.
• Synchronous and asynchronous clients covered; generated API documentation updated.
• Contract tests updated. Endpoint and wire-field fixtures are unchanged.
• Cross-SDK retry safety differences considered and preserved.
• No credentials, environment files, logs, or test output committed.

Breaking changes: callers requiring the previous response-schema checks should set _strict_response_validation=True. HTTP 529 now raises OverloadedError, directly under APIStatusError, so an InternalServerError catch alone no longer includes overloads. Invalid JSON and invalid download URLs still fail in either validation mode.