fix: preserve sub-second precision in BaseModel datetime timestamps

[wakqasahmed]

Replaces #10541, which I can't update in place — the branch there has two commits with the wrong git identity from an earlier session, and this environment blocks both rewriting that history and force-pushing over it, even to my own fork. Same diff, plus the changelog entry @michalsn asked for.

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When a model's $dateFormat is 'datetime' and the connection's dateFormat['datetime'] mask includes .v/.u (millisecond/microsecond precision), auto-generated timestamps for created_at/updated_at always rendered .000000 instead of the real sub-second value.

setDate() converted the current Time instance to a Unix timestamp integer (Time::now()->getTimestamp()) before formatting it, which discards anything below whole-second precision. timeToDate() already formats a Time object directly and correctly retains sub-second precision (Time extends DateTimeImmutable), but nothing routed the "no explicit date passed" case through it.

Changed setDate() so that when no explicit $userDate is given and the model's format is 'datetime', it calls timeToDate(Time::now()) instead of going through intToDate(). The 'int' and 'date' formats, and the case where an explicit integer timestamp is passed, are unchanged.

Added a test in GeneralModelTest that pins the current time to a value with microsecond precision and asserts both .v and .u format masks render correctly, plus the changelog entry in v4.7.5.rst.

Fixes #10540

[mergeable]

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[michalsn]

Thank you @wakqasahmed, and congrats on your first contribution to the project!