test: Move conversation ID tests to their own file

[sentrivana]

Description

For whatever reason, this collection of conversation ID tests was hidden away in test_misc.py. Move them to their own file.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

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[github-actions]

Codecov Results 📊

✅ 89982 passed | ⏭️ 6288 skipped | Total: 96270 | Pass Rate: 93.47% | Execution Time: 316m 18s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

✅ Patch coverage is 100.00%. Project has 2435 uncovered lines.
✅ Project coverage is 89.77%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    89.77%    89.77%        —%
==========================================
  Files          192       192         —
  Lines        23809     23809         —
  Branches      8218      8218         —
==========================================
+ Hits         21374     21374         —
- Misses        2435      2435         —
- Partials      1347      1347         —

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Generated by Codecov Action