chore: clean up stale and dangerous //nolint comments (#23643)

## Changes

- **Commit 1**: Remove 17 unnecessary `//nolint` directives:
  - `//nolint:varnamelen` — linter not active
  - `//nolint:unused` on exported `SlimUnsupported`
  - `//nolint:govet` in `coderd/httpmw/csrf` — no longer fires
  - `//nolint:revive` on functions refactored since the nolint was added
- `//nolint:paralleltest` citing Go 1.22 loop variable capture
(obsolete)
- Bare `//nolint` narrowed to specific `//nolint:gocritic` with
justification

- **Commit 2**: Fix root causes behind 5 dangerous nolint suppressions:
- Add `MinVersion: tls.VersionTLS12` to TLS client config (removes
`gosec` G402)
- Delete trivial unexported wrappers `apiKey()`/`normalizeProvider()` in
chatprovider (removes `revive` confusing-naming)
- Add doc comments to `StartWithAssert` and `Router` (removes `revive`
exported)
  - Rename unused parameters to `_` in integration test helpers

> 🤖 This PR was created using Coder Agents and reviewed by me.

coderd/agentapi/connectionlog_test.go

@@ -101,7 +101,6 @@ func TestConnectionLog(t *testing.T) {
 			reason: "because error says so",
 		},
 	}
-	//nolint:paralleltest // No longer necessary to reinitialise the variable tt.
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()