## Motivation and Context
`handle_regular_request` was checking `@sessions.key?(session_id)` directly
without holding `@mutex`, while concurrent threads could modify `@sessions`
via `cleanup_session` or `handle_delete`. This created a TOCTOU race where
the check could pass but the session could be deleted before subsequent use.
The class already provides a mutex-protected `session_exists?` helper, and
`handle_get` already uses it. This change makes `handle_regular_request`
consistent with `handle_get`.
## How Has This Been Tested?
Added a test that verifies `handle_regular_request` delegates to the
mutex-protected `session_exists?` helper instead of accessing
`@sessions` directly.
All existing tests pass.
## Breaking Change
None.
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