fix: Fix ValueError in signal handling for Trino worker threads

[mbm-codes]

What this PR does / why we need it:

Guards signal.signal() calls in Query.__init__() with a threading.current_thread() is threading.main_thread() check.

When the Trino offline store is served via Arrow Flight, Query.__init__() runs in a worker thread. Python restricts signal.signal() to the main thread only, causing:
ValueError: signal only works in main thread of the main interpreter

Which issue(s) this PR fixes:

Fixes #6424

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests

Misc

Added regression tests covering both main thread and worker thread scenarios.
Signal handling is preserved for CLI/interactive usage. Worker thread usage silently skips signal registration, which is the correct behavior when running under Arrow Flight.