Preserve sub-second precision when serializing timestamps

[jonathan343]

Overview

The PHP SDK serializes request timestamps by calling DateTimeInterface::getTimestamp() (or strtotime() for strings), which returns whole seconds and drops any sub-second precision the caller provided. This affects the unixTimestamp format used by json, rest-json, and smithy-rpc-v2-cbor bodies, and the iso8601 format used by query, rest-xml, and REST querystring/URI bindings. rfc822 (HTTP-date) only supports whole seconds and is unchanged.

This PR preserves the caller's sub-second precision for unixTimestamp and iso8601. Whole-second timestamps serialize exactly as before. It is the PHP counterpart to boto/botocore#3796 and fixes the PHP equivalent of boto/botocore#3255.

Testing

Added unit tests for TimestampShape::format() / formatAsString() and end-to-end serializer tests for json, query, rest-xml, and rest-json bindings. Protocol compliance tests pass. PHPStan reports no errors on the touched files.

Reproduced the scenario from boto/botocore#3255 with KinesisVideoArchivedMedia::listFragments, capturing the serialized body with a mock handler:

<?php
require 'vendor/autoload.php';

use Aws\KinesisVideoArchivedMedia\KinesisVideoArchivedMediaClient;
use Aws\Result;
use GuzzleHttp\Promise\Create;

$client = new KinesisVideoArchivedMediaClient([
    'region' => 'us-east-1',
    'version' => 'latest',
    'credentials' => ['key' => 'AKID', 'secret' => 'SECRET'],
    // Capture the serialized request instead of sending it.
    'handler' => function ($command, $request) {
        echo "POST {$request->getUri()->getPath()}\n";
        echo "body: {$request->getBody()}\n";
        return Create::promiseFor(new Result(['Fragments' => []]));
    },
]);

$client->listFragments([
    'StreamName' => 'some-stream',
    'FragmentSelector' => [
        'FragmentSelectorType' => 'PRODUCER_TIMESTAMP',
        'TimestampRange' => [
            'StartTimestamp' => new DateTimeImmutable('2024-09-12T10:49:36.500000+00:00'),
            'EndTimestamp'   => new DateTimeImmutable('2024-09-12T10:49:38.833000+00:00'),
        ],
    ],
]);

Before (master):

POST /listFragments
body: {"StreamName":"some-stream","FragmentSelector":{"FragmentSelectorType":"PRODUCER_TIMESTAMP","TimestampRange":{"StartTimestamp":1726138176,"EndTimestamp":1726138178}}}

After:

POST /listFragments
body: {"StreamName":"some-stream","FragmentSelector":{"FragmentSelectorType":"PRODUCER_TIMESTAMP","TimestampRange":{"StartTimestamp":1726138176.5,"EndTimestamp":1726138178.833}}}

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