server: add /slots/status endpoint for secure monitoring

[Roshankumarb31]

Summary

Adds a new /slots/status endpoint that provides secure slot monitoring without exposing sensitive user data.

Motivation

The existing /slots endpoint exposes prompts and generated text, making it unsuitable for production monitoring and load balancers. This was discussed in #11040 where the maintainer suggested creating a secure alternative.

Changes

• Added new /slots/status endpoint in tools/server/server.cpp
• Returns slot state and metrics without sensitive data (prompts, generated text)
• Added API documentation in tools/server/README.md
• Added security tests to verify no data leakage

Use Cases

• Production monitoring dashboards
• Load balancers (e.g., Paddler) for capacity-aware routing
• Resource allocation and capacity planning
• Any scenario requiring slot availability without user data exposure

Testing

• Code compiles successfully with MSVC on Windows
• Server starts and endpoint is accessible
• Security tests verify no sensitive data in response

References

• Discussion: The case for making `/slots` and other monitoring endpoints stable #11040

[ggerganov]

The existing /slots endpoint exposes prompts and generated text

It does not expose such data - this was fixed: #15630

[ngxson]

Beside, the whole PR (including docs + PR description) seems to be AI-generated