feat: add Gemma 4 template support and projector capability gating

[leehack]

Summary

• add Gemma 4 chat template detection, rendering, thinking parsing, and tool-call parsing support in the core template engine
• add Gemma 4 E2B to the chat app download library and gate multimodal attachment support from real projector capabilities instead of assuming any loaded mmproj supports audio
• document the current Gemma 4 GGUF projector behavior, including the vision-only runtime path exposed by the current llama.cpp mtmd integration

Validation

• dart analyze
• dart test test/unit/core/template/chat_format_test.dart test/unit/core/template/chat_template_engine_test.dart test/unit/core/template/jinja/jinja_analyzer_test.dart test/unit/core/template/handlers/gemma4_handler_test.dart
• flutter test test/assistant_output_service_test.dart
• flutter analyze

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.00000% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.92%. Comparing base (140314b) to head (fe93010).

Files with missing lines	Patch %	Lines
lib/src/core/template/handlers/gemma4_handler.dart	89.47%	18 Missing ⚠️
lib/src/backends/llama_cpp/llama_cpp_service.dart	0.00%	16 Missing ⚠️
lib/src/backends/llama_cpp/worker.dart	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   76.77%   76.92%   +0.14%     
==========================================
  Files          67       68       +1     
  Lines        8399     8592     +193     
==========================================
+ Hits         6448     6609     +161     
- Misses       1951     1983      +32     

Flag	Coverage Δ
unittests	76.92% <81.00%> (+0.14%)	⬆️

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