fix(coding-agent): default openai provider to gpt-6.1-sol (#11467)

The `openai` provider still defaulted to `gpt-5.5`. The same default
applies to API-key logins and to "Sign in with ChatGPT", so logging in
through ChatGPT selected `openai/gpt-5.5`. The reporter says that model
no longer works with a ChatGPT subscription. The `openai-codex` and
`github-copilot` providers already default to `gpt-6.1-sol`.

This change sets `defaultModelPerProvider.openai` to `gpt-6.1-sol` in
`packages/coding-agent/src/core/model-resolver.ts`. The new value
applies in three places: the model picked after `/login` when no model
was set, the fallback model at startup, and the default model for
`--provider openai`. API-key users get the new default too.

I did not test against a real ChatGPT account, so the claim that the
subscription rejects `gpt-5.5` is not checked here.

Testing:
- Updated the assertion in `test/model-resolver.test.ts` ("openai
defaults track current models"). It now expects `gpt-6.1-sol`; the
model-resolver tests pass.
- `npm run check` passes.
- `pi --list-models gpt-6.1` still lists `openai/gpt-6.1-sol`, so the
new default is in the catalog.

Fixes #11442

Requested by @badlogic

Co-authored-by: triagent <triagent@localhost>

packages/coding-agent/src/core/model-resolver.ts

@@ -21,7 +21,7 @@ export const defaultModelPerProvider: Partial<Record<KnownProvider, string>> = {
 	"amazon-bedrock": "us.anthropic.claude-opus-4-6-v1",
 	"ant-ling": "Ring-2.6-1T",
 	anthropic: "claude-opus-4-8",
-	openai: "gpt-5.5",
+	openai: "gpt-6.1-sol",
 	azure: "gpt-5.4",
 	"openai-codex": "gpt-6.1-sol",
 	radius: "balanced",

packages/coding-agent/test/model-resolver.test.ts

@@ -698,7 +698,7 @@ describe("resolveCliModel", () => {
 
 describe("default model selection", () => {
 	test("openai defaults track current models", () => {
-		expect(defaultModelPerProvider.openai).toBe("gpt-5.5");
+		expect(defaultModelPerProvider.openai).toBe("gpt-6.1-sol");
 		expect(defaultModelPerProvider["openai-codex"]).toBe("gpt-6.1-sol");
 	});