fix(session): handle missing binary attachments

[dannyward630]

Summary

• handle binary attachment read failures with the same recoverable flow as text and directory attachments
• publish a session error and add a synthetic read-failure message instead of converting the failure into a defect

Closes #32777

Verification

• bun run typecheck from packages/opencode
• full pre-push bun turbo typecheck

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a highly related PR:

Potential Duplicate:

• PR fix(opencode): handle missing binary file attachments gracefully #32788: fix(opencode): handle missing binary file attachments gracefully
  fix(opencode): handle missing binary file attachments gracefully #32788

This PR appears to address the same issue as PR #32889. Both are about handling missing or unreadable binary file attachments gracefully. This could be a duplicate or closely related work that should be reviewed together.