feat(items): add filters (pin, city, ulb_lgd, digipin) and strengthen tests

[alpha912]

Summary

• Add optional filters to /collections/addresses/items: pin, city, ulb_lgd, digipin.
• Keep existing params (limit, offset, bbox) intact; response remains GeoJSON with pagination links.
• Add tests: media type correctness, pagination links, PIN filtering, invalid bbox handling. Add conftest.py to make imports robust.

Rationale

• Enables simple attribute filtering without breaking API compatibility.
• Improves confidence via HTTP-level tests, aligning with OGC API - Features usage patterns.

Acceptance Criteria

• GET /collections/addresses/items?pin=560008 returns only features with pin=560008.
• city, ulb_lgd, digipin filters work as described (case-insensitive for city).
• Content-Type for items is application/geo+json.
• Invalid bbox returns HTTP 400 with error JSON.
• Pagination self and next/prev links computed correctly.
• All tests pass: pytest in api-server.

Security & Performance

• In-memory filtering; no persistence layer changes. No new external deps.
• Inputs are validated via FastAPI; bbox parsing guarded with 400 on invalid.

Docs

• Parameters are self-documented in OpenAPI via FastAPI (no manual spec edits required). README unchanged.

Breaking Changes

• None.

Migration Notes

• None required. Clients can start using new filters when desired.

Linked Issues

• closes #N/A

Screenshots / Logs

• 5 passed in api-server tests locally.

Changelog Entry

• feat: items endpoint supports attribute filters (pin, city, ulb_lgd, digipin).

Reviewers

• @BharatAddress/maintainers

Labels

• enhancement, api, tests