test: add vitest configuration with initial test suite

[rollroyces]

Summary

Adds comprehensive test infrastructure to PilotDeck:

• Vitest configuration with coverage reporting
• Tests for core error model (normalizeModelError)
• Tests for agent protocol errors and runtime helpers
• Tests for tool call collection and result pairing
• Tests for channel adapter interface and WebhookSessionMapper
• TurnInputProcessor unit tests

New Test Files

File	Tests
tests/model/errors/normalizeModelError.test.ts	17
tests/agent/protocol/errors.test.ts	10
tests/agent/loop/collectToolCalls.test.ts	4
tests/agent/loop/ensureToolResultPairing.test.ts	2
tests/agent/turn/TurnInputProcessor.test.ts	3
tests/adapters/channel/ChannelAdapter.test.ts	3
tests/adapters/channel/WebhookSessionMapper.test.ts	6

Changes

• Updated vitest.config.js with globals, coverage, and test inclusion
• Added test:vitest, test:watch, test:coverage scripts to package.json
• Added vitest devDependency

Impact

Establishes the testing foundation for the entire project, enabling:

• CI verification of all PRs via npm run test:vitest
• Regression prevention through unit tests
• Documentation of expected behavior via tests