fix(rum): convert relative time strings in sessions commands

[platinummonkey]

Summary

Fixes issue #37 where pup rum sessions list and pup rum sessions search commands fail with 400 errors when using relative time formats like "1h". The commands were passing raw time strings directly to the Datadog RUM API, which expects date math format ("now-1h"), ISO 8601 timestamps, or Unix milliseconds.

Changes

• Time conversion (rum.go:665-678, 713-726): Convert relative time strings (e.g., "1h", "30m") to Unix milliseconds using parseTimeString(), matching the behavior of logs commands
• Early validation (rum.go:665, 713): Validate time parameters before creating API client for faster feedback and better user experience
• Improved error messages (rum.go:668, 673, 694, 716, 721, 735): Add comprehensive error messages with:
  • Supported format examples (relative, absolute, "now")
  • Request details (parsed timestamps, original input, limit)
  • Troubleshooting tips (time range validity, permissions, data existence)
• Test coverage (rum_test.go:213-381): Add 8 new test cases across 2 test functions:
  • TestRumSessionsList_TimeConversion: 4 test cases for list command
  • TestRumSessionsSearch_TimeConversion: 4 test cases for search command
  • Tests verify both valid time conversions and invalid time error handling

Test Plan

• All existing RUM tests pass
• New time conversion tests pass (8/8 test cases)
• Full test suite passes with race detection
• Build succeeds without errors
• Behavior now matches logs commands (cmd/logs_simple.go)

Before/After

Before:

$ pup rum sessions search --query "@type:view" --from "1h" --limit 3
Error: failed to search RUM sessions: 400 Bad Request (status: 400)

After:

$ pup rum sessions search --query "@type:view" --from "1h" --limit 3
# Works correctly - "1h" is converted to Unix milliseconds

Related Issues

Closes #37

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🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 80.4%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		80.4%

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📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit ae447c1}