Send reasoning content back to the model across turns via the reasoning_content API field (#21036)

* webui: send reasoning_content back to model in context

Preserve assistant reasoning across turns by extracting it from
internal tags and sending it as a separate reasoning_content field
in the API payload. The server and Jinja templates handle native
formatting (e.g. <think> tags for Qwen, GLM, DeepSeek...).

Adds "Exclude reasoning from context" toggle in Settings > Developer
(off by default, so reasoning is preserved). Includes unit tests.

* webui: add syncable parameter for excludeReasoningFromContext

* chore: update webui build output

tools/server/webui/src/lib/constants/settings-keys.ts

@@ -54,6 +54,7 @@ export const SETTINGS_KEYS = {
 	SHOW_TOOL_CALL_IN_PROGRESS: 'showToolCallInProgress',
 	// Developer
 	DISABLE_REASONING_PARSING: 'disableReasoningParsing',
+	EXCLUDE_REASONING_FROM_CONTEXT: 'excludeReasoningFromContext',
 	SHOW_RAW_OUTPUT_SWITCH: 'showRawOutputSwitch',
 	CUSTOM: 'custom'
 } as const;