fix(bindings): reject same-instance callback re-entry

[chaliy]

Summary

• reject same-instance external_handler re-entry in the Python bindings before it can block on the interpreter mutex
• reject same-instance onOutput re-entry in the JS bindings, including pre-obtained fs() handles and sync executeSync() callbacks
• add regression coverage for both binding paths and document the JS callback contract clearly

Root Cause

Both bindings delivered user callbacks while the same Bash instance was still mid-execution and still owned the interpreter/runtime access needed by live methods. If the callback re-entered the same instance through execute*, read_file / readFile, or fs(), the binding tried to reacquire that state and either hung or panicked instead of failing cleanly.

Validation

• just pre-pr
• node --test crates/bashkit-js/__test__/runtime-compat/streaming-output.test.mjs
• targeted Python binding checks for re-entrant handler access and existing external handler coverage

Closes #1320

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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