[Platform] Add PartialJsonParser for streaming structured output

[wachterjohannes]

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

This PR adds Symfony\AI\Platform\StructuredOutput\Streaming\PartialJsonParser, a static, dependency-free utility that recovers the largest valid structure from an incomplete JSON string. This unblocks rendering partial objects from streamed structured output before the model finishes emitting them.

Algorithm

PartialJsonParser::parse() attempts a strict json_decode first; on failure it applies best-effort fixes in order and tries again:

1. Strip trailing commas before ] / }.
2. Close unclosed strings by tracking " and \ byte-wise (UTF-8 safe because both are single-byte).
3. Fix incomplete values at the tail:
   • dangling colon → append null
   • partial literals (tru, fals, nul, ...) → complete them
   • trailing comma at EOF → strip it
4. Close unclosed { / [ by walking the string with a stack that ignores chars inside strings.

If still invalid, returns null and populates the &$errorMessage reference with json_last_error_msg().

Usage

use Symfony\AI\Platform\StructuredOutput\Streaming\PartialJsonParser;

$buffer = '';
foreach ($chunks as $chunk) {
    $buffer .= $chunk;
    $partial = PartialJsonParser::parse($buffer);

    if (null !== $partial) {
        // render the partial structure
    }
}

Notes

• Ported from ModelflowAi\Chat\Util\Json\OptimisticJsonParser
• No platform wiring changes — follow-up PRs can wire it into StreamResult / structured-output streaming.
• composer test, vendor/bin/phpstan analyse, and php-cs-fixer pass cleanly on the new files.