ci(test): add race detector to CI workflow and concurrent parse test coverage

[AdamMagued]

Summary

• Add -race flag to go test in GitHub Actions CI workflow (.github/workflows/ci.yml).
• Always clone RegexDefinitions in New() to prevent concurrent writes on shared regex pointers during initialization with custom definitions.
• Protect individual parse methods (ParseUserAgent, ParseOs, ParseDevice) with reader locks against concurrent sorting passes.
• Use atomic.StoreUint64 on misses counters in checkAndSort to prevent data races against atomic increments.
• Add concurrent stress tests in uaparser/race_test.go covering shared parser usage, individual parser calls, periodic sorting, and concurrent initialization.

Fixes #104

Test Plan

• Ran go test -race ./... -v and go vet ./... in isolated container sandbox across all unit and concurrent stress tests.
• Verified all tests pass with zero data race warnings.