fix: minja upper/lower filter fails on array type values

[gerstnr]

Overview

When using Gemma 4 models with tool calling (tested Gemma-4-31B-It, unsloth gguf), requests to /v1/chat/completions or /v1/responses fail with a 500 error:

While executing FilterExpression at line 18, column 34 in source:
...if -%}
{%- if value['type'] | upper == 'STRING' -%}
^
Error: Unknown (built-in) filter 'upper' for type Array

This happens because Gemma 4's chat template (both the GGUF-embedded template and the new gemma4.jinja template from #21326) uses value['type'] | upper to check JSON Schema parameter types. Minja's value_array_t had no upper (or lower) method, causing the exception.

Reported in a comment on #21326: #21326 (comment)

Fix

Add upper and lower to value_array_t::get_builtins() in common/jinja/value.cpp, returning an empty string — the same fallback behavior already used by value_undefined_t.

Additional information

ch10299342 proposed fixing the template itself in #21326.

Valid, would preserve type-specific formatting for nullable params. The runtime fix is simpler and more defensive.
The two approaches are not mutually exclusive — a template fix could be applied on top.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: yes, root cause analysis and search for existing PRs

[ch10299342]

I'm going to close my PR this is better.

[CISC]

This is unfortunately still not quite right, I will make a PR with the proper fix.

BTW, the template does not seem to correctly handle when value['type'] is an array anyway, so it might need fixing. cc/ @ngxson @osanseviero

[CISC]

Proper jinja fix in #21370, at least from a compatibility viewpoint, the template still requires fixing IMO.

[gerstnr]

Thanks @CISC!