Document tool call syntax highlighting

[evalstate]

Summary

This PR documents syntax-highlighted tool call rendering in fast-agent.

What changed

• Added Function Tools docs for structured function_tools display metadata
• Documented support for:
  • variant: code
  • code_arg
  • language
• Added AgentCard / ToolCard docs for structured function_tools
• Added logging/config docs explaining when highlighted tool rendering appears
• Clarified that shell tool calls are highlighted automatically when tool display is enabled
• Clarified that this is display-only behavior and does not affect tool execution

Documented behavior

Structured function_tools entries can opt into code-style tool rendering:

function_tools:
  - entrypoint: tools.py:run_query
    variant: code
    code_arg: code
    language: python

Defaults for variant: code:

• code_arg: code
• language: python

Tool display behavior:

• shell tools are highlighted automatically (bash, pwsh, etc.)
• local function tools with variant: code render the selected argument as highlighted code
• non-code arguments appear as footer metadata
• rendering is gated by logger.show_tools
• there is no separate global tool-highlighting mode flag

Files updated

• docs/agents/function_tools.md
• docs/agents/defining.md
• docs/ref/agent_cards.md
• docs/ref/config_file.md

Validation

• Built docs successfully with:
  • uv run mkdocs build -q

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