fix: strict RFC 2397 regex in _parse_base64_data_uri to reject SSE data

[MoonSangJin]

Summary

• _parse_base64_data_uri previously used a loose startswith("data:") check, which misidentified SSE data (e.g., "data: {'foo': 'bar'}") as base64 data URIs
• Replace manual parsing with a strict RFC 2397 regex requiring the full data:[<mediatype>][;params];base64,<data> format
• Non-matching inputs now return (None, None) with an error log for debugging, while SSE data is no longer falsely decoded as media

Test plan

• Added 9 test cases in tests/test_issue_5659.py covering SSE data, valid data URIs, MIME params, missing MIME type, empty/invalid strings
• All existing tests/test_media.py unit tests pass
• ruff check, ruff format, mypy all pass

Closes langfuse/langfuse#5659

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