Add turn completion usage statistics logging and display

[Copilot]

Implements comprehensive usage statistics that are automatically displayed and logged after each turn completes, addressing the need for better visibility into token consumption and costs.

What's Added

Visual Display in TUI:
After each turn completes, users now see a formatted summary including:

• 📊 Turn Completed header with current turn statistics
• 🔄 This Turn section showing input/output/reasoning tokens and estimated cost
• 📈 Session Total section showing cumulative usage and context window percentage

Session Logging:
Structured JSON logs are written to the session log file containing:

• Detailed token counts (input, cached, output, reasoning) for both turn and session
• Estimated cost calculations based on typical model pricing
• Context window usage metrics
• Session ID for correlation

Example Output

📊  Turn Completed
🔄  This Turn
  • Input: 282 (+ 13473 cached)
  • Output: 2709
  • Reasoning: 2176
  • Turn Total: 2991
  • Estimated Cost: ~$0.1710

📈  Session Total
  • Session ID: c7df96da-daec-4fe9-aed9-3cd19b7a6192
  • Total Input: 61473 (+ 423266 cached)
  • Total Output: 7412
  • Session Total: 68885
  • Context Usage: 487223/400000 (122%)

Implementation Details

• Minimal Integration: Hooks into existing TaskCompleteEvent handling without disrupting current flow
• Cost Estimation: Uses placeholder pricing (~$0.03/$0.06 per 1K tokens) that can be made model-specific later
• Smart Display: Only shows statistics when non-zero token usage occurs
• Existing Infrastructure: Leverages current token tracking and session logging systems
• Test Coverage: Updated snapshot test to include new output format

The feature provides immediate visibility into token consumption patterns and helps users understand the cost implications of their interactions with Codex.

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[Copilot]

Pull Request Overview

This PR implements comprehensive usage statistics tracking that automatically displays turn completion metrics in the TUI and logs structured data to session files, providing visibility into token consumption and estimated costs.

• Adds visual display of token usage and cost estimates after each turn completion
• Implements structured JSON logging of detailed usage statistics to session logs
• Integrates seamlessly with existing token tracking infrastructure

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
codex-rs/tui/tests/fixtures/ideal-binary-response.txt	Updates test fixture to include new turn completion statistics display format
codex-rs/tui/src/session_log.rs	Adds logging function for turn completion statistics to session log
codex-rs/tui/src/history_cell.rs	Implements display formatting and cost estimation for turn completion statistics
codex-rs/tui/src/chatwidget.rs	Integrates turn completion statistics into the chat workflow and event handling

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