fix(rivetkit): spawn the local Engine on the endpoint's port

[eersnington]

Description

If you set endpoint to a local URL, RivetKit still spawned the Engine on port 6420. So the client waited on the port you asked for, and the health check failed.

endpoint: "http://127.0.0.1:54250"
├── client waits on    :54250
└── Engine listens on  :6420   <- hard-coded default

The fix: engineHost and enginePort stay unset unless you set them, so the Engine falls back to the endpoint's port.

• Nothing set: still 6420. Unchanged.
• Explicit enginePort: used as before. Unchanged.
• Only an endpoint: the Engine now listens on that endpoint's port. This never worked before.

This only affects local dev and tests. Deployed apps never spawn an Engine.

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?

# Before, RivetKit asked for port 54250:

$ pnpm exec vitest run tests/napi-runtime-integration.test.ts
target=rivetkit_napi::registry message="serving native registry" endpoint=http://127.0.0.1:54250/
target=rivetkit_engine_process message="spawned engine process (intentionally orphaned, will outlive this process)" pid=11147
[Error: {"code":"health_check_failed","group":"engine","message":"Engine health check failed after 14 attempts: error sending request for url (http://127.0.0.1:54250/health)"}]
 × native NAPI runtime integration > runs a TS actor through registry, NAPI, core, envoy, and engine

# The Engine's own log from that run shows where it bound:

 message="loaded config" config="Root { ... guard: Some(Guard { host: Some(127.0.0.1), port: Some(6420), ...
 message="HTTP server listening on 127.0.0.1:6420"

# After:

$ pnpm exec vitest run tests/napi-runtime-integration.test.ts
 ✓ native NAPI runtime integration > runs a TS actor through registry, NAPI, core, envoy, and engine  6825ms
 Test Files  1 passed (1)

message="loaded config" config="Root { ... guard: Some(Guard { host: Some(127.0.0.1), port: Some(54308), ...
message="HTTP server listening on 127.0.0.1:54308"

# Also:

$ pnpm exec vitest run tests/registry-constructor.test.ts
 Tests  6 passed (6)

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