Add comprehensive test coverage improvements

Improved overall test coverage from 37.9% to 51.0% (+13.1 percentage points)

New test files:
- internal/observability/metrics_test.go (18 test functions)
- internal/observability/tracing_test.go (11 test functions)
- internal/observability/provider_wrapper_test.go (12 test functions)
- internal/conversation/sql_store_test.go (16 test functions)
- internal/conversation/redis_store_test.go (15 test functions)

Test helper utilities:
- internal/observability/testing.go
- internal/conversation/testing.go

Coverage improvements by package:
- internal/conversation: 0% → 66.0% (+66.0%)
- internal/observability: 0% → 34.5% (+34.5%)

Test infrastructure:
- Added miniredis/v2 for Redis store testing
- Added prometheus/testutil for metrics testing

Total: ~2,000 lines of test code, 72 new test functions

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

go.sum

@@ -16,6 +16,8 @@ github.com/Azure/azure-sdk-for-go/sdk/internal v1.11.2/go.mod h1:XtLgD3ZD34DAaVI
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@@ -71,8 +73,8 @@ github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMyw
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@@ -143,6 +149,8 @@ github.com/tidwall/pretty v1.2.1 h1:qjsOFOWWQl+N3RsoF5/ssm1pHmJJwhjlSbZ51I6wMl4=
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@@ -167,6 +175,8 @@ go.uber.org/atomic v1.11.0 h1:ZvwS0R+56ePWxUNi+Atn9dWONBPp/AUETXlHW0DxSjE=
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@@ -234,13 +244,13 @@ google.golang.org/protobuf v1.22.0/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2
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