Python: [Feature]: Single shared data-URI parser and media-type sniffer used by all packages #8913

Description

Problem

Data-URI parsing and media-type detection are implemented several times, and each copy has been fixed separately:

• A2A: Python: Python A2A rejects data URIs with media type parameters #6820 / Python: accept A2A data URI media parameters #6818 (media-type parameters)
• AG-UI: Python: AG-UI state extraction ignores parameterized JSON data URIs #6902 / Python: accept AG-UI state data URI parameters #6905 (parameterized JSON data URIs)
• Gemini: Python: Fix GeminiChatClient dropping image/file content #6751; malformed URI: Python: malformed data_uri should not leak IndexError #6917; Python: reject non-base64 data URIs in detect_media_type_from_base64 #6629 (closed, non-base64 URIs)
• Core is still affected: Python: [Bug]: Content.from_uri rejects valid non-base64 data URIs with parameters and drops the media type #8905 (_validate_uri rejects data:text/plain;charset=utf-8,... and drops the media type) and Python: [Bug]: detect_media_type_from_base64 misclassifies BMP/SVG and misses MPEG-2 audio #8906 (loose BMP/SVG sniffing, missed MPEG-2 audio, non-validated base64).

Proposal

1. Add one internal helper in core, e.g. parse_data_uri(uri) -> DataUri(media_type, params, is_base64, data). It follows RFC 2397: default media type text/plain;charset=US-ASCII, arbitrary key=value params, base64 only when the last param is base64, percent-decoding for non-base64, and a clear ValueError / ContentError on malformed input.
2. Add one sniffer with strict signatures: BMP DIB-header check, SVG requiring an <svg root, the MPEG frame-sync mask, WebP/WAV RIFF subtype, and b64decode(validate=True).
3. Replace the ad-hoc data: handling found in about 15 modules (A2A, hosting-A2A, AG-UI, Gemini, OpenAI Responses/Chat Completions, core MCP, hosting-MCP, DevUI, Foundry hosting, Content Understanding, skills) with the shared helper.
4. Add a shared table-driven test corpus (valid and invalid URIs, magic-byte samples) run once in core.

Expected behavior

Every package accepts and rejects exactly the same data URIs and reports the same media type.

Code Sample

from agent_framework._types import parse_data_uri

u = parse_data_uri("data:text/plain;charset=utf-8,hello%20world")
u.media_type   # "text/plain"
u.params       # {"charset": "utf-8"}
u.data         # b"hello world"

Language/SDK

Python