fix(rivetkit-core): route serverless framework requests when the base path is root

[Saisharathchandranandnetha]

Description

Closes #5755

• The serverless listener now handles its own routes (/, /start, /health, /metadata, /metrics) when serverless.basePath is "/" instead of forwarding them to the application handler.
• Added a test for the root base path.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• cargo test -p rivetkit-core --lib listener_reserves_framework_routes_for_root_base_path fails on upstream/main with root base path should reserve /start and passes with this change.
• cargo test -p rivetkit-core --lib -- serverless:: passes (11 tests).
• cargo check -p rivetkit-core passes. cargo clippy -p rivetkit-core -- -W warnings reports nothing in the changed files.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[the-company-company]

✅ No issues found

_{Reviewed commit 3cb4edc.}