feat: v1/messages support, part 3 (server endpoints): add v1/messages endpoints to NeMo server

[RobGeada]

Description

Adds v1/messages support to the NeMo Guardrails server, as well as a v1/messages/checks endpoint, where /v1/messages is to /v1/messages/checks as /v1/chat/completions is to /v1/checks.

• Add /v1/messages and a checks endpoint that accept Anthropic Messages API format requests, with request/response schemas and SSE streaming that emits tool_use blocks.
• Adds the optional anthropic dependency to the server, all, and est_integration extras, since the server schemas depend on the anthropic SDK types.
• Adds documentation for the endpoints and additions to openapi spec

Note: Depends on:

• Part 1: feat: v1/messages support, part 1 (types): add reasoning field to ChatMessage #2282
• Part 2: feat: v1/messages support, part 2 (client): add native Anthropic v1/messages client #2283

Related Issue(s)

#2226

Verification

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• No AI tools were used.
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Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

[github-actions]

PR merge guidance

@RobGeada thanks for the PR. GitHub is currently blocking merge for one or more repository requirements:

• This branch has merge conflicts with develop. Please rebase your branch on the latest develop, resolve the conflicts locally, and force-push the updated branch.
• 3 commits do not have a verified signature (26e6b0e, 8f415a4, 778f511). Please sign the commits and force-push the updated branch.

Relevant guide:

• Signed commits: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md#commit-signing
• Contribution guide: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md