test: improve code coverage with unit tests

[bug-ops]

Summary

• Add comprehensive unit tests for hdbconnect-mcp modules
• Add CodSpeed performance badge to README
• Increase test coverage from ~83% to ~87%

New tests added

• config/builder.rs: Full coverage for all setter methods
• config/runtime.rs: RuntimeConfig reload and diff tests
• auth/config.rs: JWT config, tenant config, RBAC config tests
• helpers.rs: hdb_value_to_json edge cases, cached_or_fetch tests
• security/query_guard.rs: Comprehensive query validation tests
• observability/metrics.rs: Record functions smoke tests
• observability/mod.rs: Tracing initialization tests

Coverage analysis

Current coverage: 87.01% (was ~83%)
Target: 90%

Remaining uncovered code is primarily integration code requiring live HANA/HTTP connections:

• server.rs (MCP server implementation)
• pool.rs (connection pooling)
• transport/*.rs (HTTP/stdio transport)
• auth/oidc.rs, auth/jwks.rs (OIDC/JWKS integration)

Test plan

• All 1107 tests pass
• Coverage improved
• No regressions in existing functionality

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 13.54%

_{Comparing feature/improve-test-coverage-phase3 (2387343) with main (56cc23d)}

Summary

⚡ 1 improved benchmark
✅ 136 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	get_no_ttl_check	6.6 µs	5.8 µs	+13.54%

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.58960% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/hdbconnect-mcp/src/security/query_guard.rs	97.72%	2 Missing ⚠️
crates/hdbconnect-mcp/src/config/runtime.rs	98.66%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   84.09%   86.64%   +2.54%     
==========================================
  Files          63       63              
  Lines       12618    13349     +731     
==========================================
+ Hits        10611    11566     +955     
+ Misses       2007     1783     -224     

Flag	Coverage Δ
hdbconnect-arrow	86.59% <99.58%> (+2.56%)	⬆️
hdbconnect-mcp	86.59% <99.58%> (+2.56%)	⬆️
python	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rates/hdbconnect-arrow/src/conversion/processor.rs	97.49% <ø> (ø)
crates/hdbconnect-arrow/src/test_utils.rs	100.00% <100.00%> (+31.05%)	⬆️
crates/hdbconnect-mcp/src/auth/config.rs	100.00% <100.00%> (+23.44%)	⬆️
crates/hdbconnect-mcp/src/config/builder.rs	100.00% <100.00%> (+9.58%)	⬆️
crates/hdbconnect-mcp/src/config/mod.rs	95.12% <100.00%> (+37.22%)	⬆️
crates/hdbconnect-mcp/src/helpers.rs	84.91% <100.00%> (+10.58%)	⬆️
crates/hdbconnect-mcp/src/observability/metrics.rs	80.79% <100.00%> (+67.75%)	⬆️
crates/hdbconnect-mcp/src/observability/mod.rs	92.64% <100.00%> (+7.79%)	⬆️
crates/hdbconnect-mcp/src/config/runtime.rs	92.47% <98.66%> (+7.78%)	⬆️
crates/hdbconnect-mcp/src/security/query_guard.rs	97.71% <97.72%> (+28.74%)	⬆️

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