Options, Meta APIs: Remove orphaned transient timeout rows on delete and cleanup

[HasnainAshfaq]

Fixes the two-part bug reported in #65863. This PR addresses both halves of the report — unlike the previously submitted PR #13101 which only fixed the first half.

The problem:

set_transient() writes the timeout row before the value row in two separate, non-transactional statements. If a request dies between those two writes, the timeout row is committed but the value row never arrives.

This creates a permanently stuck orphan row because:

1. delete_transient() conditions cleanup of the timeout row on the value row deletion succeeding. With no value row, it returns false and leaves the timeout row forever.
2. delete_expired_transients() uses a multi-table DELETE that joins from the value row side, so an orphaned timeout row has no a side to join from and is never seen — regardless of its timestamp.

The same applies to delete_site_transient().

What this PR does:

Fix 1 — delete_transient() and delete_site_transient():
Removes the if ( $result ) guard so the timeout row is always deleted when requested, whether or not the value row existed.

Fix 2 — delete_expired_transients():
Adds a LEFT JOIN DELETE statement after each existing cleanup query to remove expired orphaned timeout rows (timeout exists, value row is absent). Covers all three variants: single-site transients, single-site site transients, and multisite sitemeta.

Tests:

• test_delete_transient_removes_orphaned_timeout_row() — directly simulates the orphan and asserts delete_transient() cleans it up.
• test_delete_expired_transients_removes_orphaned_expired_timeout_row() — expired orphan is removed by the cleanup job.
• test_delete_expired_transients_keeps_orphaned_future_timeout_row() — non-expired orphan is left alone.

SQL offset note:
_transient_timeout_ is 19 characters, so SUBSTRING(a.option_name, 20) extracts the transient name. _site_transient_timeout_ is 24 characters, so SUBSTRING(a.option_name, 25) is used for the site transient variants. These match the proposed queries in ticket comment #3 from the reporter.

See https://core.trac.wordpress.org/ticket/65863

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