loop: Add focused regression tests for generateSessionUuid

[sleuth-loop-engineer]

Objective

GitHub issue #6: Add focused regression tests for generateSessionUuid without modifying production code.

Planned work

• Add focused regression tests for generateSessionUuid — Create test/utils/session_uuid_test.dart (following the existing test/utils convention, e.g. importing package:sleuth/src/utils/... and using flutter_test) that adds a group for 'generateSessionUuid' with two deterministic, network-free tests: (1) a generated value matches an anchored RFC 4122 v4 UUID expression — full-length 8-4-4-4-12 hex shape with version nibble '4' after the second hyphen and variant nibble [89ab] after the third hyphen — using expect(value, matches(RegExp(...))); and (2) two successive generated UUID values are different via expect(first, isNot(second)). Do not touch lib/ or any production code; this task produces exactly one repository file change.

Acceptance criteria

• AC-1 Add test/utils/session_uuid_test.dart and keep the delivery test-only.
• AC-2 Generated values match an anchored RFC 4122 v4 UUID expression with version nibble 4 and variant nibble [89ab].
• AC-3 Two successive generated UUID values are different.
• AC-4 Focused test, analyzer, and formatter checks pass.

Delivery provenance

Merge run: 858c1d5a-2273-53bd-b362-0c9a2b04fbd1
Accepted local head: 04ec7860187950cb1d7f3c7f26c59e8e8decf6b5
Published head: d516130eddc958e6edf0ea8ff8fc1a75d6ae564c

[Harrys76]

Please strengthen this regression test by generating a batch of 20 session UUIDs and asserting that every generated value matches the RFC 4122 version-4 format, while retaining an explicit assertion that two successive values are different. Keep the change test-only.