Dispatch inbound WeChat image and video media

[octo-patch]

Reason: The WeChat callback recognizes image and video messages, but the handler discards them before agent dispatch.

• Forward supported image and video URLs into the inbound agent context with matching MIME metadata.
• Preserve callback content types and continue rejecting unsupported or unsafe media references.
• Add focused coverage for callback normalization and inbound media context construction.

Checks:

• ./node_modules/.bin/tsx --test test/bot-media.test.ts

AI disclosure: This pull request was prepared with Codex assistance for code changes and tests. A human maintainer must review and understand the patch before merge.