perf: clean up GTM init listeners after first successful initialization

[Nsanjayboruds]

Description

This PR fixes #7590

This change improves GTM lazy-init behavior in gatsby-browser.js by ensuring all interaction listeners are cleaned up immediately after the first successful GTM initialization.

What changed

• Added GTM_DELAY_MS constant for clear delay units.
• Added centralized listener management:
  • addInitListeners()
  • removeInitListeners()
• Replaced per-event removal logic with full listener cleanup after successful initGTM().
• Kept one-time init guard (window.gtmDidInit) and existing dataLayer pageview push behavior unchanged.
• Updated delay comment to reflect actual behavior in milliseconds.

Notes for Reviewers

Behavioral outcome:

• GTM still initializes exactly once.
• If GTM initializes via timeout first, interaction listeners are now removed immediately (no stale listeners left behind).
• If GTM initializes via user interaction first, same immediate cleanup occurs.

Validation performed:

• npx eslint gatsby-browser.js (passes; only environment-level browserslist update notices)
• Manual code-path verification for timeout and interaction-triggered init flows.

Signed commits

• Yes, I signed my commits.

[github-actions]

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[Bhumikagarggg]

@Nsanjayboruds Thank you for your contribution! Let's discuss this during the website call tomorrow at 5:30 PM IST | 7 AM CST Add it as an agenda item to the meeting minutes, if you would 🙂

[Nsanjayboruds]

Hey @CodexRaunak, could you please review this PR when you get a chance? Thank you!