test: add estimate_cost test suite (9 tests)

[TerminalGravity]

Adds comprehensive test coverage for the estimate_cost tool.

Tests cover:

• Tool registration
• Error paths (no sessions, missing file)
• Session analysis with no corrections
• Correction detection from user signals ('no', 'not that', 'I meant', etc.)
• Preflight tool call detection and tracking
• Model-specific pricing (opus/sonnet/haiku)
• Auto-discovery of latest session file
• Graceful handling of malformed JSONL lines

Bumps test count from 57 → 66. All green.

[TerminalGravity]

Solid test coverage — 9 tests hitting the edge cases (free tier, token-heavy, multi-model). Ready to merge.