Add MCP SLI availability tool

[slok-bot]

Summary

• add the get_slo_sli_availability_range MCP tool
• return compressed availability series with RFC3339 start timestamp and shared step metadata
• reuse the backend SLI availability range use case through MCP

AI

AI-assistant: openai/gpt-5.4
Usage: assisted

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.65217% with 74 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.30%. Comparing base (636daa0) to head (07b31fb).

Files with missing lines	Patch %	Lines
internal/http/mcp/tools/toolsmock/mocks.go	0.00%	65 Missing ⚠️
...l/http/mcp/tools/get_slo_sli_availability_range.go	87.23%	3 Missing and 3 partials ⚠️
internal/http/mcp/mcp.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #825      +/-   ##
==========================================
+ Coverage   31.18%   31.30%   +0.11%     
==========================================
  Files         148      149       +1     
  Lines        9132     9228      +96     
==========================================
+ Hits         2848     2889      +41     
- Misses       6036     6088      +52     
- Partials      248      251       +3     

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