refactor: replace deprecated 'context.master' with 'context.primary' for inclusive language (#33068)

[Row-heat]

Summary

This pull request updates all references of context.master to context.primary across the AMPHTML codebase as part of an inclusive language refactor, in alignment with issue #33068.

The following changes were made:

• ampcontext-integration.js: Refactored the IntegrationAmpContext class and masterSelection() function.
• 3p.js: Updated computeInMasterFrame() usage from global.context.master to global.context.primary.
• test-3p.js: Updated all test setup references using context.master.

No functional or behavioral changes were introduced. This is a semantic change to align with inclusive coding standards.

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Motivation

The use of terms like "master" can be problematic and non-inclusive. This refactor improves code readability and ensures the project aligns with modern inclusive language practices.

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Notes

• Confirmed that context.slave is not used anywhere in the codebase.
• Ensured all references were updated safely.
• All existing test cases were adjusted to use the new terminology where necessary.
• CI should pass as no logic was altered.

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Closes: #33068

[CLAassistant]

All committers have signed the CLA.

[Row-heat]

Hi @caroqliu 👋
This PR updates variable names to promote inclusive terminology. All relevant instances of context.master have been replaced with context.primary. Please review when convenient. Thanks!