fix: raise HTTPError on failed API responses instead of returning None

[EarningsCall]

Summary

• get_symbols_v2(), get_sp500_companies_txt_file(), get_events(), and get_exchanges_json() now raise HTTPError on failed API responses instead of silently returning the raw response. Previously, non-200 responses were passed through to callers, which would then crash with AttributeError (e.g., calling .split() on a non-text response) or return corrupt data.
• _get_events() in Company now handles HTTPError (returns [] on 404), consistent with get_transcript(), download_audio_file(), and download_slide_deck().
• Added return type hints to all public functions in api.py
• Added error-path unit tests (404/500) with status code assertions for all fixed functions
• Fixed missing @responses.activate decorator on test_download_audio_file_missing_params_raises_value_error
• Bumps version to 2.0.1

Test plan

• All 89 tests pass (pytest)
• New tests assert HTTPError is raised with correct status code in message ("500 Server Error", "404 Client Error")
• company.events() returns [] on 404, raises on 500
• Existing test_get_sp_500_companies_failed_request updated to expect HTTPError instead of empty list
• Coverage increased +0.8% to 97.48%

🤖 Generated with Claude Code

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[EarningsCall]

Bugbot finding addressed: _get_events() now catches HTTPError and returns [] on 404, consistent with get_transcript, download_audio_file, and download_slide_deck. Added tests for both 404 and 500 cases.