fix: register missing Situation endpoint route (#798)

[tejasva-vardhan]

Overview

This PR resolves #798 by properly registering the GET /api/where/situation/{id} route in the REST API. While the handler logic was present, it was not wired to the router, causing a 404/HTML fallback when requesting real-time service alerts.

Changes

• Route Registration: Registered the missing endpoint in internal/restapi/routes.go.
• Logic Implementation: Re-implemented the situationHandler to correctly fetch and format service alerts using api.extractAndValidateID.
• Data Access: Added GetAllAlerts() in internal/gtfs/realtime.go to provide access to the in-memory GTFS-RT alert store.
• Testing: Added unit tests in internal/restapi/situation_handler_test.go (verified with go test).

Verification (Before vs After)

To ensure full parity with the legacy OBA Java API, I verified the fix using both the maglev-validator and manual browser testing.

1. Maglev Validator Comparison

Local Maglev (localhost:4000) vs Production Java API (api.pugetsound.onebusaway.org).

Stage	Comparison Result
Before (Gap)
After (Fixed)

2. Browser Manual Testing

Endpoint: /api/where/situation/1_84714.json?key=test

Stage	Response Screenshot
Before (404)
After (200 OK)

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Note for Reviewers: - Only core logic files for the fix are included in this PR.

• Windows-specific driver changes (modernc.org/sqlite) and local performance bypasses were intentionally excluded to maintain upstream standards.

[tejasva-vardhan]

@aaronbrethorst @Ahmedhossamdev would love any review on this!