Tests: Use assertSame() in WP_AI_Client_Prompt_Builder tests.

[sagarsdeshmukh]

Migrates assertEquals() to assertSame() in tests/phpunit/tests/ai-client/wpAiClientPromptBuilder.php.

All 78 assertEquals() calls compare scalar values (strings, ints, floats, or string arrays), so strict === comparison is appropriate. Two assertions comparing timeout values were updated to use float literals (30.0, 45.0) to match the actual return type of getTimeout().

172 tests, 439 assertions — all passing.

See https://core.trac.wordpress.org/ticket/64324

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Props sagardeshmukh.

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[SergeyBiryukov]

Thanks for the PR! Merged in r62248.