fix: add error handling to RPC system to prevent crashes on session abort

[oorestisime]

What does this PR do?

Fixes a crash that occurs when aborting a session with double-escape. The app would crash with "Unexpected end of JSON input" error.

Fixes #7946

An example of error

error/crash [09:17:24] [LOG] 'bootstrapping'
[09:17:25] [LOG] 'resolveSystemTheme'
[09:17:25] [LOG] '{"type":"home"}'
[09:17:26] [LOG] [
      '#1d1f21', '#cc6666',
      '#b5bd68', '#f0c674',
      '#81a2be', '#b294bb',
      '#8abeb7', '#c5c8c6',
      '#666666', '#d54e53',
      '#b9ca4a', '#e7c547',
      '#7aa6da', '#c397d8',
      '#70c0b1', '#eaeaea'
    ]
[09:17:49] [LOG] 'navigate' { type: 'session', sessionID: 'ses_44eee8c1cffeyYbm3ENuvzhYkm' }
[09:17:49] [LOG] '{"type":"session","sessionID":"ses_44eee8c1cffeyYbm3ENuvzhYkm"}'
[09:17:53] [LOG] 'TSWorker:' 'Loading from local path: /$bunfs/root/tree-sitter-markdown-411r6y9b.wasm'
[09:17:53] [LOG] 'TSWorker:' 'Loading from local path: /$bunfs/root/highlights-r812a2qc.scm'
[09:17:53] [LOG] 'TSWorker:' 'Loading from local path: /$bunfs/root/injections-73j83es3.scm'
[09:17:53] [LOG] 'TSWorker:' 'Loading from local path: /$bunfs/root/tree-sitter-markdown_inline-j5349f42.wasm'
[09:17:53] [LOG] 'TSWorker:' 'Loading from local path: /$bunfs/root/highlights-x6tmsnaa.scm'
[09:25:00] [ERROR] Error: Unexpected end of JSON input
    SyntaxError: Unexpected end of JSON input
        at <parse> (:0)
        at json (unknown)
        at <anonymous> (../sdk/js/src/v2/gen/client/client.gen.ts:167:33)
        at processTicksAndRejections (native:7:39) 

Changes:

• Add try/catch in RPC listener to send error responses back to client instead of leaving promises hanging
• Handle rpc.error messages in client to reject promises properly
• Add .catch() to session.abort call to prevent unhandled rejections
• Return JSON from /session/:sessionID/prompt instead of a streaming response to avoid empty bodies on abort
• Fire-and-forget /session/:sessionID/prompt_async with a 204 no-content response and server-side error logging

How did you verify your code works?

1. Started opencode TUI
2. Started an agent task
3. Pressed escape twice to abort mid-work
4. Verified the app no longer crashes with JSON parse errors

Note: I have not rerun the flow after the latest server response adjustments in this environment.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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No duplicate PRs found