fix(models): fallback to application config default context size in /v1/models/capabilities (#12202)

[PINYOPATTANAWASANPORN]

Description

This PR fixes #12202.

When a model has no explicit context_size configured in its YAML configuration, /v1/models/capabilities was unconditionally defaulting to DefaultContextSize (4096), ignoring the application-wide default context size configured under Settings -> Performance -> Default Context Size (appConfig.ContextSize).

Key Changes:

• In ListModelCapabilitiesEndpoint (core/http/endpoints/openai/list_capabilities.go), if a model's cfg.ContextSize is unset/zero and appConfig.ContextSize > 0, fall back to appConfig.ContextSize so client capability discovery respects the configured default.
• Added comprehensive unit test in core/http/endpoints/openai/list_capabilities_test.go asserting that when appConf.ContextSize is configured, models with unconfigured context size report appConf.ContextSize.

Notes for Reviewers

• Surgical 38-line diff (+16 / 0 lines) with normalized LF line endings.
• Backward compatible: if appConfig.ContextSize is 0 or unset, backend.EffectiveRequestContextSize(cfg) cleanly retains the 4096 default fallback.

Signed commits

• Yes, I signed my commits.
• Documentation updated (docs/content/) for user-facing changes, or not applicable