fix(tui_rpc): surface the cause of an internal_error, not just the code

[arelchan]

Summary

A -32603 reached the user as exactly this, and nothing else:

error: [rpc -32603] internal_error

No cause, no file, no log path. The information existed the whole time and was discarded
three separate times:

1. The dispatcher dropped detail. raven/tui_rpc/dispatcher.py used
   if exc.data is not None: ... elif exc.detail: ..., so a raiser that set both lost the
   detail. _build_tui_agent_loop always sets both.
2. turn.send emitted only {code, message} for a latched init crash, although the
   ErrorEvent schema already has a detail field and chatStream.onError already renders
   it.
3. The client formatted [rpc <code>] <message>, where message is a fixed code name
   (internal_error), and every call site prints err.message.

So an AgentLoop that cannot start produced a dead end. A real instance: a config file
containing a key the running branch does not know about fails validation, _build_tui_agent_loop
raises InternalError with the full pydantic message and log_path, and the TUI showed the
line above.

The fix keeps message protocol-faithful and puts the cause where callers already look:

• error_data(exc) in raven/tui_rpc/errors.py folds detail into data, and both the
  dispatcher and the turn error events use it, so the two paths carry the same context.
• turn.send passes the init-crash detail plus its log_path into the emitted event.
• formatRpcError in ui-tui/src/rpc/errors.ts builds the message from detail,
  exception_message or reason, and appends the log path. A multi-line cause (a config
  error listing each offending field) keeps its line breaks below the summary line.

Same failure after the change, rendered by feeding a real dispatcher frame through the client
path:

error: [rpc -32603] internal_error:
Config at /Users/admin/.raven/config.json fails schema validation:
1 validation error for Config
subagents
  Extra inputs are not permitted
(details in ~/.raven/logs/tui.log)

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

uv run pytest tests/ -k "tui_rpc or dispatcher or tui_bootstrap" -> 523 passed. New cases:
the dispatcher keeps detail next to reason and log_path in one frame; turn.send emits
the build-error cause with its log path; and it omits detail entirely when the error has no
cause to report.

npx vitest run in ui-tui/ -> 86 files, 988 tests passed, including six new
rpcErrorFromFrame cases: a bare frame keeps the old one-line form; data context is
surfaced with the log path; exception_message and reason are read when detail is absent;
a multi-line cause keeps its shape; non-object or blank data cannot corrupt the message; and
typed subclass selection plus raw data access are unchanged.

npm run type-check, npx eslint, npx prettier --check, ruff check, ruff format --check
all clean.

End to end, the exact production shape: an InternalError raised the way
_build_tui_agent_loop raises it was dispatched to a JSON-RPC frame, and that frame was fed
through rpcErrorFromFrame with tsx, producing the output shown above.

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Notes: error.code and error.message are unchanged on the wire, so nothing that branches on
them is affected; the added data.detail and payload.detail are both already in the schema.
err.message gains a suffix, which the one message-matching call site
(SESSION_BUSY_RE in useSubmission.ts) is unaffected by, since it matches text that still
appears. Detail text originates from server-side exceptions and is already written to
~/.raven/logs/tui.log, so this exposes nothing new to the client; the client accepts it only
when it is a string and ignores any other shape. Rollback is a revert of this commit.

Related Issues

N/A