test: let LLM budget failures fail CI

[ling-senpeng13]

Summary

• remove the Anthropic usage-limit exception handling introduced in test: skip agent e2e when LLM budget is exhausted #445
• let provider budget exhaustion fail Agent E2E normally
• require API budget problems to be corrected manually instead of hidden by test skips

Behavior

• missing provider credentials may still skip tests that cannot run
• an exhausted provider budget is a CI failure
• unrelated provider and test failures continue to propagate normally

Validation

• targeted Agent E2E test collection: 3 tests collected
• python -m compileall -q e2e/test_suite1_basic_validation.py e2e/test_suite25_media_input.py
• git diff --check

This supersedes the budget-skip behavior added by #445.

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[ling-senpeng13]

Closing because LLM budget exhaustion should be fixed manually rather than handled by test logic.