test: add unit tests for get_the_terms() object cache handling

[SainathPoojary]

The ticket reports that get_the_terms() returns false instead of the correct terms when get_object_term_cache() returns an empty array.

Investigation shows the false === $terms check is correct. It distinguishes a cache miss from a valid "no terms" state ([]). When terms are assigned, wp_set_object_terms() properly clears the cache so get_the_terms() will query the database on the next call. The reported bug is likely a test environment configuration issue.

As requested, this PR adds unit tests to verify the expected cache behavior:

1. Cache primed with [] before terms assigned → terms assigned → returns correct terms.
2. Cache primed with [] for a post with no terms → returns false without a DB query.
3. Cache primed after term assignment → returns correct terms from cache.

All tests pass on current trunk with no source change needed.

Trac ticket: https://core.trac.wordpress.org/ticket/49799

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Used for: Investigating the cache flow and generating initial unit tests. Implementation and testing were reviewed and edited by me.

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