fix(session): respect model media capabilities

[opencode-agent]

What does this PR do?

Prevents unsupported image and PDF attachments from remaining inside tool results and poisoning later history replay.

Media support now honors both capability representations: the legacy attachment flag and the modality-specific capabilities.input.image / pdf flags. If either representation enables the relevant medium, the model is treated as supporting it. Provider-specific rules still decide whether supported media can remain inside a tool result; otherwise it is routed through a synthetic user message.

The same compatibility rule is applied when unsupported user media is transformed, so legacy attachment configuration continues to work after extraction.

Verification

• bun test ./test/session/message-v2.test.ts — 39 pass, 0 fail
• Scoped oxlint — 0 errors (existing warnings only)
• git diff --check
• bun run typecheck was attempted twice but did not complete within the host timeout; it produced no diagnostics before timeout.

Requested by: @rekram1-node (Aiden Cline via Slack)

[github-actions]

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