Add failing test for reverse mapped this leak (#62779)

[priyankagnana]

Related to #62779
This PR adds a failing compiler test for issue #62779.

Summary
The test demonstrates that this inside a reverse-mapped type leaks a generic key type and causes incorrect inference (T[string]) instead of the concrete property return type.

Files added

• tests/cases/compiler/reverseMappedThisLeak.ts
• tests/baselines/reference/reverseMappedThisLeak.errors.txt
• tests/baselines/reference/reverseMappedThisLeak.js
• tests/baselines/reference/reverseMappedThisLeak.types
• tests/baselines/reference/reverseMappedThisLeak.symbols

Reproduction
The test is the minimal reproduction from the issue and is runnable with the existing test harness.

Notes

• This PR does not include a compiler fix — only a failing test + baselines to help maintainers reproduce and track the bug.
• To reproduce locally:
  • npm test -- tests/cases/compiler/reverseMappedThisLeak.ts (should pass once baselines are in reference/)
  • npm test -- --generateBaseline was used earlier to create these baselines.

(Please let me know if maintainers want a smaller repro or further variations.)

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[jakebailey]

We don't need a PR for the failing test alone. The issue already has it, so we should add this when the issue is fixed.