FIX: validate cancel_job delete flag type

[biru-codeastromer]

Closes #359

Summary

• validate the delete flag type in cancel_job_tool
• reject non-boolean values before any job record is removed
• add a regression test for delete="false"

Problem

cancel_job_tool treats the delete parameter as a plain truthy/falsy value.
That means malformed MCP calls like:

cancel_job_tool(job_id, delete="false")

still delete job records, because non-empty strings are truthy in Python.

I reproduced this on current code by creating a completed job and calling cancel_job_tool(job_id, delete="false"), which returned success and removed the job.

Fix

Add a strict boolean validation guard near the start of cancel_job_tool:

• if delete is not a real bool, return a validation error
• do not delete anything in that case

Tests

Added regression coverage to ensure:

• cancel_job_tool(job_id, delete="false") returns an error
• the job record remains present after the invalid call

Verification

• ruff check .
• ruff format --check .
• pytest -q