fix: make tracker.stop() idempotent

[sohammishra864-wq]

Body:

Description

Make tracker.stop() idempotent so calling it multiple times does not produce duplicate emissions rows or trigger duplicate handler cleanup.

Related Issue

Closes #1307

Motivation and Context

When using the tracker as a context manager and also calling stop() explicitly to capture the return value, __exit__ calls stop() a second time. The second call ran the full measurement-persist-exit cycle again, writing a duplicate CSV row and double-calling every output handler's `exit()

The root cause: _start_time was never cleared after stopping, so the existing guard never triggered. Additionally, the lock release ran before the guard, causing a redundant
release on re-entry.

How Has This Been Tested?

• Verified calling stop() twice returns the same emissions value without writing a
  duplicate row
• Verified with block + explicit stop() produces a single CSV row

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or ting functionality to change)

AI Usage Disclosure

• AI-vibecoded
• AI-generated
• AI-assisted
• No AI used

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the **docs/hent.
• I have added tests to cover my changes.
• All new and existing tests passed.

[SaboniAmine]

Thanks for your contribution!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.64%. Comparing base (3fcd4dc) to head (186dc4e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1408      +/-   ##
==========================================
- Coverage   91.68%   91.64%   -0.04%     
==========================================
  Files          49       49              
  Lines        5136     5137       +1     
==========================================
- Hits         4709     4708       -1     
- Misses        427      429       +2     

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