fix: Gemini system messages, tool result formatting, and argument key symbolization

[ejwhite7]

Problem

Three issues in ruby-pi cause the agent loop to fail with HTTP 400 errors when tools are involved, which is the root cause of the ongoing olli-social avatar chat failures (see ejwhite7/olli-social-app PR #78 discussion).

Issue 1: Gemini rejects system messages in contents

The Loop#build_llm_messages prepends a { role: :system } message. Gemini's API does not accept system as a role in the contents array — it requires system instructions to be sent via the dedicated systemInstruction field. This causes an immediate HTTP 400 on every Gemini call when a system prompt is set.

Issue 2: Gemini rejects role: "tool" in contents

When the agent loop executes tool calls, it adds results as role: :tool messages to state. On the next think() call, format_message maps these to { role: "tool", parts: [{ text: ... }] } — but Gemini only accepts user and model roles. Tool results must be sent as functionResponse parts. This causes HTTP 400 on the second LLM call in any tool-using conversation.

Issue 3: String-keyed arguments break tool implementations

LLM providers return tool arguments as JSON with string keys ({"query" => "..."}) but Ruby tool implementations expect symbol keys (args[:query]). This causes tools to silently receive nil for all parameters, leading to incorrect results or errors.

Fix

lib/ruby_pi/llm/gemini.rb

• build_request_body: Extracts role: :system messages from the messages array and sets body[:systemInstruction] with their content. Only non-system messages are included in contents.
• format_message: Detects role: "tool" messages and converts them to Gemini's functionResponse format ({ role: "user", parts: [{ functionResponse: { name: ..., response: { result: ... } } }] }). Falls back to plain text with role: "user" when tool name is missing.

lib/ruby_pi/tools/executor.rb

• execute_single: Calls deep_symbolize_keys on arguments before passing them to the tool block.
• deep_symbolize_keys (new private method): Recursively converts all string keys to symbols in hashes, including nested hashes and arrays.

Tests

Added specs for all three fixes:

• Gemini: verifies systemInstruction is set and system messages excluded from contents
• Gemini: verifies tool messages are formatted as functionResponse
• Executor: verifies string keys are symbolized (flat, nested, array cases)
• Executor: verifies already-symbolized keys pass through without error

All 334 specs pass.