fix(anthropic-provider): resolved Anthropic request and stream handling

- Added a 4-slot concurrency limiter for Anthropic multi-image resize work.
- Prioritized client-owned request fields and `claudeCodeVersion`-based `User-Agent` headers.
- Ignored terminal `message_delta` envelopes and treated missing `usage`/`delta` payloads as non-fatal.
- Stopped ping-only streams from consuming first-event watchdog logic and preserved timeout retries.

packages/ai/test/github-copilot-anthropic-auth.test.ts

@@ -224,6 +224,38 @@ describe("Anthropic Copilot auth config", () => {
 		expect(result.baseURL).toBe("http://127.0.0.1:8317");
 	});
 
+	it("sends Content-Type and anthropic-version on Copilot anthropic requests", () => {
+		const result = buildAnthropicClientOptions({
+			model: makeCopilotClaudeModel(),
+			apiKey: "ghu_test",
+			extraBetas: [],
+			stream: true,
+			dynamicHeaders: {},
+		});
+
+		// The client posts JSON.stringify(params); without these the request goes
+		// out with no Content-Type at all (Bun does not default it for string
+		// bodies when a plain headers object is supplied).
+		expect(result.defaultHeaders["Content-Type"]).toBe("application/json");
+		expect(result.defaultHeaders["anthropic-version"]).toBe("2023-06-01");
+	});
+
+	it("merges Copilot headers case-insensitively so auth headers cannot duplicate", () => {
+		const result = buildAnthropicClientOptions({
+			model: { ...makeCopilotClaudeModel(), headers: { ...OPENCODE_HEADERS, authorization: "Bearer override" } },
+			apiKey: "ghu_test",
+			extraBetas: [],
+			stream: true,
+			dynamicHeaders: {},
+		});
+
+		// A miscased duplicate would survive Object.assign and the Headers
+		// constructor then joins both values comma-separated on the wire.
+		const authKeys = Object.keys(result.defaultHeaders).filter(key => key.toLowerCase() === "authorization");
+		expect(authKeys).toHaveLength(1);
+		expect(result.defaultHeaders[authKeys[0]]).toBe("Bearer override");
+	});
+
 	it("builds anthropic auth URLs from the normalized service root", () => {
 		const url = buildAnthropicUrl({
 			apiKey: "test-key",