feat(sdk/python): add media message parts support

[alexander-akhmetov]

Note

Medium Risk
Touches generation export, validation, and content-capture paths where incorrect URL handling could leak or drop multimodal payloads; scope is additive and well-tested but affects exported message shape.

Overview
Adds media as a first-class message part in the Python SDK, aligned with Go generation export and cross-SDK hook behavior.

The public surface gains PartKind.MEDIA, a Media model (kind, url, mime_type, name), and media_part(). Proto mapping emits a media oneof when a payload is present and drops media parts with no media object (same as Go) instead of sending empty text. Validation requires a non-blank media.url (with an exception when content was stripped for metadata-only capture) and allows media on any message role.

Metadata-only stripping now clears media.url while keeping kind, MIME type, and name; secret redaction explicitly leaves media URLs untouched (only metadata-only clears them). Hook request serialization omits parts the evaluate endpoint cannot read, including media; hook response parsing drops kind: media parts. Docs and Go conformance comments are updated to note Python can hold media like Go, while JS has no media type.

Tests cover proto mapping, validation, content capture, and hooks conformance.

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