fix(assistant): normalize requestId and add input guards in AgentStreamRequestRegistry

[hsusul]

Fixes #1919

Problem

In src/helpers/agentStreamRequestRegistry.js:

• begin(senderId, requestId) validated that requestId.trim() was non-empty, but stored the untrimmed string. If a request was registered with surrounding whitespace and cancelled without it (or vice versa), the controller lookup missed and the active stream failed to abort.
• cancel, complete, and cancelSender did not guard against invalid types (e.g. non-integer senderId or non-string requestId).

Solution

• Normalized requestId with .trim() consistently across begin, cancel, and complete.
• Added input type guards for senderId and requestId across registry methods.
• Added unit tests in test/helpers/agentStreamRequestRegistry.test.js.

Verification

• node --test test/helpers/agentStreamRequestRegistry.test.js (passes, 7/7 tests)
• npm run typecheck (passes, 0 errors)
• npm run lint (passes, 0 errors)
• npm run i18n:check (passes)
• npm run build:renderer (passes)
• git diff --check (clean)