Enable anthropic count_tokens api to be proxied also

[simcop2387]

This should hopefully fix any last compatibility issues with #475

Summary by CodeRabbit

• New Features

  • Added support for the v1/messages/count_tokens endpoint, enabling users to count tokens for message operations through the OpenAI API integration.

• Documentation

  • Updated API endpoint reference documentation to include the newly supported token counting operation.

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[coderabbitai]

Walkthrough

This pull request adds support for a new /v1/messages/count_tokens endpoint in the message proxy system. The change includes documentation in README.md and endpoint routing configuration in proxy/proxymanager.go, using the same authentication middleware and handler as existing message-related endpoints.

Changes

Cohort / File(s)	Summary
Documentation README.md	Adds reference to v1/messages/count_tokens endpoint in the OpenAI API supported endpoints list
Endpoint Configuration proxy/proxymanager.go	Registers new /v1/messages/count_tokens route with existing message endpoint authentication middleware and handler

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• proxy: add support for anthropic v1/messages api #417: Modifies proxy route handling for message-related endpoints in proxy/proxymanager.go alongside this PR's new /v1/messages/count_tokens route
• proxy: add /v1/responses and /v1/audio/voices endpoints #448: Adds new proxy HTTP routes in proxy/proxymanager.go using the same pattern of wiring endpoints to auth middleware and existing handlers

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: enabling the Anthropic count_tokens API to be proxied, which is confirmed by the changes in README.md and proxy/proxymanager.go.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

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