fix(runner): preserve exact aggregate intervals

[linkdata]

Summary

• preserve exact aggregate interval durations in outbound HTTP headers
• format intervals as ISO-8601 durations instead of rounded minute counts
• add duration and header regression coverage

Tests

• go test ./pkg/runner ./...

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Reviewing files that changed from the base of the PR and between b615285 and 09c4ca6.

📒 Files selected for processing (2)

• pkg/runner/aggregate_sender.go
• pkg/runner/aggregate_sender_test.go

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[linkdata]

Additional context from comment files

These notes were split from the local markdown comment files and attached here because they describe this PR's change.

Exact Aggregate Intervals

• Bug: Histogram upload metadata truncated the interval start to the minute and rounded the duration to whole minutes.
• Impact: Partial histogram files, including shutdown flushes or any second-precision filename interval, could be reported to aggrec with the wrong Aggregate-Interval, such as a 45-second interval becoming PT1M or PT0M.
• Fix: The sender now preserves the exact start timestamp and formats the duration as an ISO 8601 duration with second precision when needed.
• Reasoning: The filename parser already preserves start/stop seconds, and aggrec accepts ISO 8601 start/duration intervals, so the upload metadata should reflect the file's actual interval rather than a rounded minute bucket.
• Tests: Added duration formatting coverage and an aggregate sender test that posts to a local HTTP server and verifies the exact Aggregate-Interval header.