Inconsistent session-not-found error envelope: pause_execution and list_threads throw McpError while sibling tools return structured success:false

[debugmcpdev]

Found by an osoji audit panel review (two independent adjudications agreeing, at commit 2f2782c).

Behavior

For the identical condition — a request carrying an unknown/stale sessionId — tools diverge in error envelope:

• pause_execution: handlePause (src/server.ts ~L1278-1293) re-throws validateSession's plain McpError (~L276, ~L1285) → protocol-level error.
• list_threads: handleListThreads (~L1295-1305) re-throws the same way (~L1302).
• Every sibling control tool (set_breakpoint, start_debugging, step/continue: ~L812-823, ~L858-866, ~L1082-1092) and every other retrieval tool (get_variables/get_stack_trace/get_scopes: ~L1132-1141, ~L1155-1169, ~L1181-1190) converts the identical failure to a structured {success: false, error: ...} result.

Evidence it's drift, not design

The typed-error conversion branches inside handlePause/handleListThreads are dead code: SessionNotFoundError extends McpError (src/errors/debug-errors.ts ~L44) and is re-thrown one line earlier. docs/error-handling-guide.md's own continue example (~L89-97) contradicts actual behavior.

Suggested fix

Declare one envelope contract for session-not-found and align both handlers to the conversion path used by the other seven tools (the dead typed-error branches then become live).

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