refactor(codersdk): use ReadBodyAsJSON in typed endpoints - #27857

Migrate chat endpoint response decoding to `ReadBodyAsJSON` and
consolidate `ReadBodyAsError` construction through `newResponseError`,
so empty-body and non-JSON errors consistently include the request
method and URL.

Stacked on #27857, with the lint rule following in #27859. Refs #27044.

Reviewed and updated by Coder Agents on behalf of @dylanhuff-at-coder.

Add a ruleguard rule forbidding direct
`json.NewDecoder(res.Body).Decode(...)` on `*http.Response` in codersdk
packages, so new typed endpoints use `codersdk.ReadBodyAsJSON` and keep
returning structured errors for non-JSON bodies. The rule matches both
the chained call form and decoders assigned to a variable first.

Intentional raw-body paths carry documented `//nolint:gocritic`
exceptions: the 16 agent-direct HTTP decodes in
`workspacesdk/agentconn.go` route through a single `decodeAgentJSON`
helper (agent-direct over tailnet, so `ReadBodyAsJSON`'s reverse
proxy/SSO error guidance does not apply), and the Azure IMDS
attested-document decode in `agentsdk/azure.go` keeps an inline
exception.

The two `UseNumber` decoders in `licenses.go` are migrated to a new
`codersdk.ReadBodyAsJSONUseNumber`, so `coder licenses add/list` also
return structured errors for non-JSON bodies instead of `invalid
character '<' looking for beginning of value`.

Note for local verification: golangci-lint caches results, so run
`golangci-lint cache clean` after modifying `scripts/rules.go` or the
rule may silently not fire.

Final PR of the stack on #27804, #27857, and #27858. Refs #27044.


Stack plan

Inventory (full-tree audit): 280 migratable call sites across 47 files;
17 excluded (16 agent-direct HTTP sites in `workspacesdk/agentconn.go`,
1 Azure IMDS decode in `agentsdk/azure.go`).

1. **#27857** `refactor(codersdk): use ReadBodyAsJSON in typed
endpoints`: mechanical migration of all sites except `chats.go` (224
sites, 46 files).
2. **#27858** `refactor(codersdk): use shared error helpers in chat
endpoints`: migrate the 56 `chats.go` sites and consolidate the
duplicated `readRawBodyAsError`/`newResponseError` helpers onto the
shared `client.go` error path, with regression tests for the 409
usage-limit flow.
3. **#27859** `chore: forbid direct response body JSON decode in
codersdk`: ruleguard rule with documented exceptions for the intentional
raw-body paths, plus `ReadBodyAsJSONUseNumber` for the `licenses.go`
decoders.



Reviewed and updated by Coder Agents on behalf of @dylanhuff-at-coder.