test: add unit tests for server application

[Aniketh-74]

Addresses #44

Adds a comprehensive test suite for server/app.py with 30 test cases:

• MilvusSearchClient (7 tests): lazy init, connection reuse,
  encoder caching, error recovery, result formatting, similarity
  calculation, content truncation
• execute_tool (5 tests): known/unknown tools, malformed args,
  empty results, citation deduplication
• health_check (3 tests): /health endpoint, non-health paths
• TOOLS config (3 tests): structure, required params, bounds
• SYSTEM_PROMPT (4 tests): content validation
• Message parsing (4 tests): JSON, plain text, bytes, edge cases
• Config (4 tests): default env variable values

All tests use unittest.mock — no Milvus, KServe, or LLM required.

Run: python -m pytest server/tests/test_app.py -v

[google-oss-prow]

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