Moves the fantasy pin to the tip of `coder_2_33` (`85ffbbe`). That
commit is coder/fantasy#72, which merges 87 upstream
charmbracelet/fantasy commits, v0.40.0 to v0.45.5. Between the two pins
117 files change. Upstream's own v0.40.0 to v0.45.5 change, without
tests, testdata, examples, `go.sum` and the kronk provider, is 43 files,
+2271/−928.

## What reaches chatd

chatd builds only the `anthropic`, `openai` and `openaicompat` fantasy
clients (`chatprovider.ModelFromConfig`); other vendors reach it through
the AI Gateway on one of those. So the upstream changes to the Google,
Vercel, OpenRouter, Bedrock, Azure and kronk providers, and to the
`fantasy.Agent` loop and its retries, do not run in chatd. These do:

- **Chat Completions and openaicompat streams** keep reasoning on
batched and DeepSeek-shaped chunks, keep `finish_reason: length` when
tool calls are truncated, and stop sending empty reasoning fields.
openaicompat drops assistant turns that hold only reasoning.
- **Token usage.** When an OpenAI-shaped backend reports reasoning
tokens apart from completion tokens (reasoning larger than output),
output tokens now include them, so that backend's reported output tokens
and cost rise. OpenAI's own shape, where reasoning is a subset, is
unchanged.
- **Responses.** A content-filtered turn no longer reads as a tool-call
turn.
- **Context overflow.** Baseten, Fireworks and Parasail prompt-length
errors set `ContextTooLargeErr`, which compaction acts on.
- **Anthropic.** Tool calls whose content block never closes are
reported. For Claude 5 models, thinking defaults to `display:
"summarized"` when the chat config sets none; a configured display still
wins. `TestThinkingDropBlockModel_RetriesWithEffort` now expects the
field in the retry body.
- **Anthropic keepalives.** Fantasy forwards `message_start`,
`message_delta` and unknown events as content-free `keepalive` parts.
The first one arrives before any model output, so this PR makes
`ttft_seconds` and the `time_to_first_token` stage skip them, as they
skip warnings. They still reset the stream silence guard.
`processStepStream` ignores them, and the debug recorder counts them in
its part count but records no content.
- **Tool-result media** now goes through upstream's shared
`ToolRunBuffer`, which replaces coder/fantasy#64 and does the same
thing.

#72 keeps coder's anthropic-sdk-go and openai-go forks, Go 1.26 and
kronk v1.31.5, so upstream's SDK and Go moves do not come along.

`go mod tidy` raises the modules fantasy v0.45.5 requires, among them
otel exporters v1.47, the AWS SDK, `google.golang.org/api` v0.307 and
go-openapi v1. All are upgrades.

The `go.mod` comment now names #72 as the upstream sync.

#29861 stacks on this PR to pin coder/fantasy#62.

<details>
<summary>Verification</summary>

- `go build ./...`.
- `go test` passes on every package that imports fantasy
(`coderd/x/chatd/...`), and `go test ./coderd -run
'TestChat|TestExpChat|TestMCP|TestAIBridge'` passes.
-
`TestGuardedStreamTTFTStage/WarningsAndKeepalivesThenContentObservesOnce`
fails without the TTFT change (TTFT observed 0s instead of 1s) and
passes with it.
- `make pre-commit` passes. The generated Prometheus docs pick up the
metric help change.

Not verified against live backends in this PR: the stream fixes, the
token usage change and the context-overflow patterns have no coder test
against a real provider. coder/fantasy#72 reports a local chatd run
against the AI Gateway on Anthropic, Responses, OpenAI Chat and
openaicompat.

</details>

> 🤖 This PR was created with the help of Coder Agents, and _will be_
reviewed by a human. 🏂🏻