fix(json-schema): factory backward compatibility layer

[soyuka]

link #4753

[bendavies]

@soyuka this seems to cause a break between v2.6 and v2.7
specifically

$this->assertMatchesResourceItemJsonSchema(PolicyNotice::class);

works on 2.6 but breaks on 2.7 because 2.7 tries to "guess" the operation and can't find one, so creates a new one with the wrong (empty) serialization context.

core/src/JsonSchema/SchemaFactory.php

Line 284 in 4c16efe

// Guess the operation and use the first one that matches criterias

in 2.7 i have to do this to fix it:

$this->assertMatchesResourceItemJsonSchema(PolicyNotice::class, '_api_/policy-notices/{id}.{_format}_get');

this can't be intended, surely?
can you provide any info on what i should be doing, or is this a regression/bug?