fix: use full Kiro payload for prompt token counting

[juslintek]

Problem

The prompt_tokens reported to clients (used by Claude Code's /context command) were wildly inaccurate because they were derived from Kiro's contextUsagePercentage, which returns unreliable values.

Solution

Count tokens from the complete serialized Kiro request payload using tiktoken. This includes system prompt, messages, tools, and all other payload fields — matching what actually gets sent to the API.

Changes

• Replace request_messages/request_tools params with pre-counted prompt_tokens across all streaming functions
• Count tokens from full kiro_request_body in both OpenAI and Anthropic route handlers
• Remove dependency on contextUsagePercentage for token counting
• Update tests to match new function signatures

[cla-bot]

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[juslintek]

I have read the CLA and I accept its terms