feat(useragent): include ai-agent in structured format

[amaskara-dd]

What does this PR do?

Change AI agent from space-separated suffix to structured key-value inside parentheses for better parsing compatibility.

Format change:
Before: pup/dev (go go1.25.0; os darwin; arch arm64) claude-code
After: pup/dev (go go1.25.0; os darwin; arch arm64; ai-agent claude-code)

Benefits:

• Consistent with existing metadata structure (all fields use key-value)
• Better parsing for downstream analytics

All tests pass.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

[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 d0c29ac}