common : fix tool call type detection for nullable and enum schemas (#21327)

common : fix tool call type detection for nullable and enum schemas (#21327)

* common : fix tool call type detection for nullable and enum schemas

* common, tests : fix grammar delegation for nullable/enum schemas and add tests

Fix enum type inference to scan all enum values (not just index 0) so
schemas like {"enum": [0, "celsius"]} correctly detect string type.

Fix schema_delegates in peg-parser to handle nullable type arrays
(["string", "null"]) and typeless enum schemas in raw mode, allowing
the tagged parser to use raw text instead of JSON-formatted strings.

Add test cases for Qwen3-Coder (TAG_WITH_TAGGED format):
- nullable string ["string", "null"]
- nullable string with null first ["null", "string"]
- nullable integer ["integer", "null"]
- enum without explicit type key