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 fix(providers/anthropic): convert reasoning effort to budget thinking on pre-4.6 models 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.

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