[intervals-mcp-server] Add HR and pace curve tools

[arnold-maderthaner]

Summary

Adds get_hr_curves(...) and get_pace_curves(...), analogous to get_athlete_power_curves (GET /athlete/{id}/hr-curves and /pace-curves). Useful to track aerobic progress (same pace at lower HR).

• HR: best HR for a set of durations (5s … 1h).
• Pace: best time and pace for distances (400 m … marathon), shown as min/km, or /100m for swims; optional gap=True for gradient-adjusted pace.
• Requested durations/distances missing from the curve are listed as "Not available" instead of being dropped.

Notes

• Response shapes verified with live Run and Swim requests.

Testing

• uv run --locked pytest, mypy src tests pass; ruff check passes on the changed files (the two pre-existing F401 findings in server.py and power_curves.py are untouched).
• Manually tested against a real Intervals.icu account through the MCP streamable-HTTP transport (see notes above).

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