fix: REST has no catch-all envelope for untyped top-level failures (cross-transport parity)

[numarasSigmaSoftware]

Follow-up from #1547 re-review. The "top-level / pre-handler failure surfaces the AdCP two-layer error envelope on the wire (not a transport-native error)" contract is currently graded — and on REST, implemented — only partially across transports.

State per transport

• A2A — covered. on_message_send's outer handler returns a failed Task carrying the two-layer envelope; graded by @T-UC-002-ext-nl-unsupported (A2A-only, correctly, since NL dispatch is an A2A entry point).
• MCP — envelope is carried. _translate_to_tool_error (src/core/tool_error_logging.py) runs normalize_to_adcp_error() on any exception and builds the two-layer envelope, so an untyped top-level failure still surfaces AdCPToolError with the envelope. (Message-scrub for untyped exceptions is the separate concern in fix: sanitize untyped-error messages before the buyer-facing wire/webhook #1587.)
• REST — gap. src/app.py registers exception handlers for AdCPError, ValueError, RequestValidationError, PermissionError, and ToolError, but there is no @app.exception_handler(Exception) catch-all. An untyped top-level exception (e.g. RuntimeError, a DB/OS error) escaping a REST endpoint gets FastAPI's default bare 500 with no AdCP envelope.

Proposed fix

1. Add a catch-all @app.exception_handler(Exception) in src/app.py that routes through safe_adcp_error(exc) (leak-safe: untyped → generic SERVICE_UNAVAILABLE envelope, no str(exc)) and returns _envelope_response(...), mirroring the MCP boundary. Framework exceptions with their own handlers (HTTPException, RequestValidationError) are unaffected — a generic Exception handler only catches what no more-specific handler claims.
2. Add per-transport integration tests grading "untyped top-level failure → two-layer envelope" on REST and MCP (A2A already covered), each asserting the same envelope values in that transport's shape.

Scope note

Split out of #1547 deliberately: #1547's scope is A2A error routing, and a REST catch-all changes every REST endpoint's untyped-500 shape — a cross-transport parity change that deserves its own review. Adjacent to #1587 (which scrubs the untyped-error message); the catch-all here should use the same safe_adcp_error sink, so the two are natural to land together.