feat(pkg-r): Support streaming ContentThinking

[simonpcouch]

Addresses #76, but only on the R side. Adds support for displaying ContentThinking during streaming and on reload.

Without corresponding changes in ellmer, this PR doesn't do anything; with main ellmer, shinychat behaves as it currently does, just streaming the thinking content as if it were normal assistant text content. With an incoming ellmer PR (that adds a Remotes on this PR), this is how the content streams + reloads:

ellmer-shinychat-thinking.mp4

Here's the script I was using the test in that video:

# ...load dev versions of both packages

# Anthropic with thinking ------------------------------------------
chat_anthropic <- chat_claude(
  model = "claude-sonnet-4-5-20250929",
  params = params(reasoning_tokens = 1024)
)

clipr::write_clip(
  "Write a limerick about the tidyverse, returning only the limerick."
)
chat_app(chat_anthropic)

# OpenAI with reasoning ------------------------------------------
chat_openai <- chat_openai(
  model = "gpt-5.1",
  params = params(reasoning_effort = "high")
)

chat_app(chat_openai)

# Gemini with thinking ------------------------------------------
chat_gemini <- chat_google_gemini(
  model = "gemini-3-flash-preview",
  params = params(reasoning_tokens = 1024)
)

chat_app(chat_gemini)

[simonpcouch]

Gemini likes to provide multiple sequential thinking blocks, otherwise resulting in UI like this without this change:

[simonpcouch]

I'm unable to request reviews on this repo, but cc @gadenbuie :)

[bianchenhao]

When will these codes be merged, I'm suffering this problem that each thinking content wraps lines. >_<

other attached packages:
[1] shiny_1.11.1 shinychat_0.3.0.9000 ellmer_0.4.0.9000