fix: stop sending adaptive thinking to pre-4.6 Anthropic models (#27314)

## Problem

A chat model config with `reasoning_effort` set on a pre-4.6 Anthropic
model (for example `claude-haiku-4-5`) fails every generation with HTTP
400 `adaptive thinking is not supported on this model`, surfaced in chat
as "Anthropic returned an unexpected error." The fantasy Anthropic
provider always serialized effort as `thinking: {type: "adaptive"}` plus
`output_config.effort`, a shape only Claude 4.6+ accepts.

## Changes

- Bump the coder/fantasy pin to include coder/fantasy#47: the provider
now converts effort into `{type: "enabled", budget_tokens}` on models
older than Claude 4.6, with the budget derived from the call's
`max_tokens` (aibridge-mirroring ratios, 1024-token API floor; below the
floor thinking is omitted, which keeps small-budget calls like title
generation working). Adaptive-capable models keep the current shape, and
Opus 4.5 keeps `output_config.effort` alongside the derived budget since
it supports effort without adaptive thinking. Models older than Claude
3.7 predate extended thinking, so effort sends no thinking at all there.
`minimal` is normalized to `low`, `xhigh` falls back to `max` on
adaptive models that predate the xhigh tier (Claude 4.7+); effort `none`
disables thinking, including an explicit `thinking: {type: "disabled"}`
on Claude 5+ models that otherwise run adaptive thinking by default. The
Bedrock provider wraps the Anthropic one, so both are covered, and
Vertex-style `@date` model IDs parse correctly.
- `TestActiveServer_CompactionModelOverride` previously codified the
buggy shape (asserting `output_config.effort` sent to
`claude-3-5-haiku-latest`). The summary-routing subtest is now a
three-case table: pre-thinking override models (Claude 3.5) expect no
thinking, legacy budget-thinking ones (Haiku 4.5) expect enabled
thinking with the derived budget, adaptive-capable ones still expect
`output_config.effort`.
- New regression test `TestActiveServer_AnthropicModelReasoningEffort`:
a `claude-haiku-4-5` config with `reasoning_effort` produces enabled
thinking with the derived budget and no `output_config` on the wire, and
a `claude-sonnet-5` config with effort `none` sends an explicit thinking
disable.
- `chattest.AnthropicRequest` gains a `Thinking` field so tests can
assert the thinking config.
- One-sentence note in the chatd ARCHITECTURE reasoning-effort section.

No chatd production code changes: `ApplyReasoningEffort` keeps setting
`Effort`, which is now valid for every Anthropic model.

## Validation

- `go test ./coderd/x/chatd/...` passes (19 packages).
- Fork PR validated separately: full fantasy test suite plus new
provider unit tests (version gating incl. Vertex/Bedrock IDs, budget
derivation, floor behavior, normalization, effort `none` incl. Claude 5+
disable, Opus 4.5 effort preservation, sampling-param stripping),
golangci-lint clean.

Closes
[CODAGT-812](https://linear.app/codercom/issue/CODAGT-812/reasoning-effort-on-pre-46-anthropic-models-fails-generations-with).

> This PR was authored by Mux, an AI coding agent, acting on Mike's
behalf.

go.sum

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