Sync GLM and Nano Banana API documentation

[Copilot]

Synchronizes the GLM and Nano Banana documentation with the latest upstream guides and OpenAPI specifications.

• GLM

  • Document glm-5.3, expanded message roles, reasoning controls, tool options, and response metadata
  • Update token acquisition guidance, canonical links, examples, and model recommendations

• Nano Banana

  • Add nano-banana-2-lite and official-channel model variants
  • Document multi-image billing, safety behavior, task timing fields, and 403 forbidden responses
  • Correct API links and provide a valid quick-start request

{
  "action": "generate",
  "model": "nano-banana-pro",
  "prompt": "A watercolor painting of a mountain lake at sunrise",
  "count": 1
}

• Fixes sync: update from Docs (1f8c57f docs: sync from PlatformBackend [automated]) #891

[Germey]

Superseded by new sync trigger; Copilot will open a fresh PR for the latest Docs state.