Add MCP burned budget range tool

[slok-bot]

Summary

• add the get_burned_budget_range MCP tool
• return compressed time series with RFC3339 start timestamp and shared step metadata
• explain the normalized remaining-budget semantics in the tool description

AI

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 38.04348% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.18%. Comparing base (24d9a45) to head (f0c9411).

Files with missing lines	Patch %	Lines
internal/http/mcp/tools/toolsmock/mocks.go	0.00%	46 Missing ⚠️
...rnal/http/mcp/tools/get_slo_burned_budget_range.go	81.39%	4 Missing and 4 partials ⚠️
internal/http/mcp/mcp.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #824      +/-   ##
==========================================
+ Coverage   31.11%   31.18%   +0.06%     
==========================================
  Files         147      148       +1     
  Lines        9040     9132      +92     
==========================================
+ Hits         2813     2848      +35     
- Misses       5983     6036      +53     
- Partials      244      248       +4     

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