Fix debugger hang when expanding objects with blocking property getters (#2053)

[rchiodo]

Summary

Fixes #2053.

Inspecting a LanceDBConnection in the debugger hung on Python 3.13. The root cause is not a slow repr — it's a thread deadlock during variable expansion.

At a breakpoint, debugpy suspends all threads (in multi_threads_single_notification mode), including background daemon threads. LanceDBConnection exposes @property getters such as read_consistency_interval that dispatch work to a background asyncio event loop running in a daemon thread and block on the result (asyncio.run_coroutine_threadsafe(...).result()). When the variables view expands the object, debugpy evaluates those properties on the suspended thread, which waits forever on the also-suspended background loop thread → deadlock.

This is why:

• repr(db) is fast — it never touches the properties.
• vars(db) works — it only reads the instance __dict__, no property evaluation.
• Setting db = None before the breakpoint avoids the hang.

Changes

1. Recover instead of hang (the real cure)

• New constant PYDEVD_UNBLOCK_THREADS_ON_VARIABLES_TIMEOUT (default 3.0s). When resolving a variable's children takes longer than the timeout, the other threads are resumed until the resolution finishes, then re-suspended — so a property that depends on another thread can complete instead of deadlocking.
• Reuses the existing evaluate-path anti-deadlock machinery, extracted into a shared unblock_threads_on_timeout context manager in pydevd_vars.py. _run_with_unblock_threads now uses it (evaluate-path behavior unchanged).
• Wired into internal_get_variable_json (the DAP variables request).

2. Dedupe double getattr

• DefaultResolver._get_py_dictionary previously did hasattr() + getattr(), evaluating every property/descriptor twice. It now calls getattr() a single time inside try/except AttributeError. This halves the cost (and side effects) of expanding objects with expensive properties.

Tests

• New resource _debugger_case_deadlock_thread_variables.py and test_debugger_case_deadlock_thread_variables reproduce the exact scenario (a property getter blocking on a suspended background thread). The test passes with the fix and deadlocks/fails when the timeout is disabled, proving it catches the regression.
• All existing evaluate-path deadlock/unblock tests pass (test_debugger_case_deadlock_thread_eval, test_debugger_case_breakpoint_on_unblock_thread_eval, test_debugger_case_unblock_manually, test_debugger_case_deadlock_notify_evaluate_timeout, test_debugger_case_deadlock_interrupt_thread).
• test_hasattr_failure, test_getattr_warning, and the broader variable/resolver test set pass, confirming the getattr dedupe is behavior-preserving.

All changes are in pure-Python pydevd modules (no Cython regeneration needed).