Add domain-scenario Gherkin tags to all A2A, session, and CLI feature
files (30 files) so tests can be filtered individually via behave.
- 8 A2A feature files: @a2a tag
- 7 session feature files: @session tag
- 15 CLI feature files: @cli tag
ISSUES CLOSED: #9124
Add Behave feature/step pairs that exercise uncovered branches across handlers, LSP, CLI, and service layers to reach the coverage gate.
ISSUES CLOSED: #1232