fix: resolve resource leaks and improve namespace lifecycle management

- Fix DNS resolver leaks by creating temporary resolv.conf files within the profile's runtime directory and ensuring robust cleanup.
- Fix isolation block directory leaks by explicitly removing the block directory during namespace unpinning.
- Improve namespace lifecycle management:
    - Register processes before joining an active namespace to prevent race conditions in reference counting.
    - Update `IsLastProcess` and corresponding tests to reflect the unregister-then-check cleanup flow.
- Improve test reliability and correctness:
    - Convert `TestAppRun_ProfileDirInjection` to use separate binary execution, preventing process replacement and ensuring `t.TempDir()` cleanup.
    - Replace hardcoded test configuration paths with `t.TempDir()` in `mount_leak_test.go`.
    - Implement `SetEnvOverrides` helper for cleaner environment variable management in E2E tests.
    - Improve E2E lifecycle tests with better environment handling and output redirection.

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