[Template] Support standalone tool observations

[taking-lying-flat]

What changed

• Normalize OpenAI assistant.tool_calls before direct template encoding, including messages without a content field.
• Support standalone tool observations following a user message in the Swift template backend.
• Add native standalone-observation formatting for ReAct, Hermes/Qwen, and GLM agent templates.
• Reuse the same OpenAI tool-call normalization in dataset preprocessing.

Root cause

Direct template encoding accessed content before normalizing assistant.tool_calls, and the Swift encoder could not represent a user -> tool -> assistant sequence even when the model chat template supports standalone observations.

Compatibility

Existing assistant -> tool(s) behavior is unchanged. Standalone observations remain prompt-side and are excluded from labels.

Fixes #9843
Refs #8499

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.