fix: decode URL escapes before base64 data URI payloads

[ADITYA (adity982)]

Summary

Fixes #2481.

Decode URL escapes before base64-decoding a data URI's payload. Previously, a valid escaped payload could raise binascii.Error (for example SGVsbG8%3D) or silently decode different bytes (%2B/8= produced d81ffc instead of fbff). The URL escaping applies to base64 payloads too: RFC 2397, section 3.

This preserves raw + characters (not form decoding), decodes escapes exactly once, and does not change MIME attributes or base64 validation policy. Add 18 regression cases covering padding, escaped plus/slash, partial escaping, lowercase escapes, metadata, non-base64 compatibility, Unicode and the public conversion API.

Validation

• Before the production change: 16 new tests failed / 2 passed against upstream cc0ca9edd8e23b2c7f7b778c7b148c1565730498.
• After the change: all 18 regression cases pass.
• Full core package suite on Windows / Python 3.12.10 with the repository's CI environment (PYTHONUTF8=1, GITHUB_ACTIONS=true) and constrained [all] dependencies: 890 passed, 34 skipped. External/provider tests follow the repository's existing skip rules; no provider keys were supplied.
• Separate CLI suite with the required UTF-8 setting: 50 passed.
• Black 23.7.0 check, source distribution/wheel build, focused URI-helper mypy check and git diff --check pass.

Validation boundaries: the initial run without CI settings had three Unicode CLI failures and one external speech-service assertion failure. The CLI failures disappear with the already-required UTF-8 setting; the speech failure independently reproduces in a clean, unchanged upstream worktree. The full repository type check reports 21 errors in 8 unchanged files, exactly matching the clean upstream baseline; it is not a passing type gate. The full hosted Python/OS matrix, OCR/MCP sibling packages and live external-provider behavior were not validated locally.

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