[Platform] Add DeferredResult::asPartialJsonStream() for streaming structured output

[wachterjohannes]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	-
License	MIT

Note

Depends on #2113. Merge after that one or rebase this PR once it lands.

Follow-up to #2113. The PartialJsonParser introduced there is a static utility — useful, but it leaves consumers to wire up the streaming buffer themselves. This PR adds DeferredResult::asPartialJsonStream(), which feeds the running text buffer into the parser and yields each new partial value, so the caller doesn't have to touch Object denormalization or build their own listener.

Behavior

• Accumulates TextDeltas from asTextStream() into a single buffer.
• Re-parses the buffer with PartialJsonParser after every delta.
• Yields the recovered value (array / scalar / object) only when it differs from the previous one — so whitespace-only deltas don't produce duplicate snapshots.
• Silently swallows buffers that are not recoverable yet (parser sets \$errorMessage).

Usage

\$deferred = \$platform->invoke('gpt-5-mini', \$messages, ['stream' => true]);

foreach (\$deferred->asPartialJsonStream() as \$partial) {
    // render the partial structure (array/object/scalar)
}

The return type is intentionally mixed (matches the parser) — this PR stays at the "raw JSON" layer. Typed object denormalization on top of the partial stream is a separate follow-up.

Notes

• New example at examples/platform/partial-json-stream.php prompts a model for a structured book object via streaming output and renders each snapshot as it grows.
• Docs extended under the existing "Parsing Partial JSON from Streams" section.
• CHANGELOG entry added.
• composer test and vendor/bin/phpstan analyse on the platform component pass (the existing unmatched-ignore in Schema.php is unrelated and tracked in [Platform] Remove unmatched PHPStan ignore in JsonSchema Schema attribute #2110).