fix(ensembl): implement resilient request retry, backoff, and fallback resolution (v0.5.1)

[engkinandatama]

Description

Addresses and resolves Ensembl REST service instability and timeouts observed post-e116 release (closing #4).

Key Changes

1. Adaptive Request Pipeline (EnsemblAdapter._get_json):
   • Centralized network access with automatic rate-limit token bucket acquisition.
   • 4-stage exponential backoff handling HTTP 429 (Rate Limit) and HTTP 500/502/503/504 errors.
   • Raised default Ensembl request timeout to 25.0s (RateLimitConfig.ENSEMBL_TIMEOUT_SEC).
2. Endpoint Fallback Logic:
   • fetch_by_id: Fallback gracefully from expand=1 to expand=0 when Ensembl internal server errors occur on complex gene models, recovering full transcript models via symbol lookup.
   • fetch_orthologs: If /homology/id/ endpoint encounters HTTP 500/503 ("No server available"), automatically falls back to /homology/symbol/{source}/{symbol}, guaranteeing 100% ortholog retrieval.
3. Test Suite Reactivation:
   • Removed @pytest.mark.skip from all 5 Ensembl unit and MCP integration tests in tests/test_resolver.py and tests/test_server.py.
   • All 22 resolver, server, and end-to-end pipeline tests pass cleanly without degradation.
4. Version Bump:
   • Bumped package version to 0.5.1 in pyproject.toml.

Closes #4