refactor phase 6: TRY003 narrow ignore; per-line TRY301 exception

The ARCPError exception class already carries typed structure (a
`code: ErrorCode` enum field plus a `details: dict`); the human-readable
message is intentionally a free-form string, not a per-error subclass.
TRY003 ("avoid long messages outside the exception class") doesn't fit
this design, so add it as a narrow project-level ignore with the
rationale captured inline. The same justification covers TransportClosed
("transport is closed", "websocket closed", etc.) and the two pydantic
field-validator ValueErrors in envelope.py.

For TRY301 in runtime.server cancel-handler: the raise-inside-try is
deliberate so a single except clause normalizes both validation-thrown
and explicit-raised ARCPError into one cancel.refused envelope. Add a
per-line noqa with the reason.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

pyproject.toml

@@ -73,6 +73,12 @@ select = [
 ]
 ignore = [
     "E501",  # line-too-long: handled by ruff format; surfaced cases are pydantic Field descriptions.
+    # TRY003: raise-vanilla-args. The ARCPError exception class already carries
+    # typed structure (`code: ErrorCode`, `details: dict`); the message string
+    # is the human-readable explanation. Splitting one subclass per message
+    # would add code without adding type structure. Same applies to
+    # TransportClosed and the two pydantic field-validator ValueErrors.
+    "TRY003",
 ]
 
 [tool.ruff.lint.per-file-ignores]

src/arcp/runtime/server.py

@@ -217,7 +217,7 @@ async def _handle_cancel(rt: ARCPRuntime, state: SessionState, env: Envelope) ->
                     )
                     await rt._send(state, accepted)
                 else:
-                    raise ARCPError(
+                    raise ARCPError(  # noqa: TRY301 — raise inside try is deliberate; the except below converts validation errors into a cancel.refused envelope alongside other ARCPError sources.
                         ErrorCode.UNIMPLEMENTED,
                         f"cancel target {payload.target!r} not supported in v0.1",
                     )