Preserve tool call/result roundtrip and raw payloads across Claude, Gemini and OpenAI adapters

[CJackHwang]

Motivation

• Improve handling of tool calls and tool results so roundtrip metadata and raw payloads are preserved across the Claude, Gemini and OpenAI adapters and avoid injecting legacy history markers.
• Normalize assistant tool-call representations into structured envelopes so downstream code can reliably detect tool_calls and tool messages.

Description

• Rewrote normalizeClaudeMessages to split text blocks, convert tool_use blocks into assistant tool_calls envelopes, convert tool_result blocks into tool messages, and preserve raw unknown blocks as JSON via formatClaudeBlockRaw.
• Updated tool prompt generation in buildClaudeToolPrompt to remove legacy marker instructions and keep clear instructions about not printing tool-call JSON, and added helpers normalizeClaudeToolUseToAssistant and normalizeClaudeToolResultToToolMessage.
• Enhanced OpenAI normalization in normalizeOpenAIMessagesForPrompt, splitting logic into buildAssistantContentForPrompt, buildToolContentForPrompt, and normalizeAssistantToolCallsForPrompt so assistant tool_calls and tool/function roles are preserved as tool messages with serialized envelopes.
• Adjusted responses input normalization in normalizeResponsesInputItemWithState to preserve assistant tool_calls and to include tool_call_id/name metadata for tool/function roles.
• Updated prompt rendering in MessagesPrepare to handle tool role formatting when building the final prompt.
• Added/updated tests: new Gemini tests in convert_messages_test.go, extended Claude tests in handler_util_test.go, and multiple updates to OpenAI normalization tests in message_normalize_test.go to assert preservation of tool envelopes and metadata.

Testing

• Ran unit tests with go test ./... across the repository and all tests completed successfully.
• Updated OpenAI unit tests in internal/adapter/openai/message_normalize_test.go to reflect preserved tool role and assistant tool_calls, and those tests passed.
• Added Gemini tests in internal/adapter/gemini/convert_messages_test.go and updated Claude tests in internal/adapter/claude/handler_util_test.go, and both test suites passed.

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