fix: detect additional context overflow error patterns to prevent leak to user (#20539)

* fix: detect additional context overflow error patterns to prevent leak to user

Fixes #9951

The error 'input length and max_tokens exceed context limit: 170636 +
34048 > 200000' was not caught by isContextOverflowError() and leaked
to users via formatAssistantErrorText()'s invalidRequest fallback.

Add three new patterns to isContextOverflowError():
- 'exceed context limit' (direct match)
- 'exceeds the model\'s maximum context'
- max_tokens/input length + exceed + context (compound match)

These are now rewritten to the friendly context overflow message.

* Overflow: add regression tests and changelog credits

* Update CHANGELOG.md

* Update pi-embedded-helpers.isbillingerrormessage.test.ts

---------

Co-authored-by: echoVic <AkiraVic@outlook.com>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts

@@ -189,6 +189,18 @@ describe("isContextOverflowError", () => {
     }
   });
 
+  it("matches exceed/context/max_tokens overflow variants", () => {
+    const samples = [
+      "input length and max_tokens exceed context limit (i.e 156321 + 48384 > 200000)",
+      "This request exceeds the model's maximum context length",
+      "LLM request rejected: max_tokens would exceed context window",
+      "input length would exceed context budget for this model",
+    ];
+    for (const sample of samples) {
+      expect(isContextOverflowError(sample)).toBe(true);
+    }
+  });
+
   it("ignores normal conversation text mentioning context overflow", () => {
     // These are legitimate conversation snippets, not error messages
     expect(isContextOverflowError("Let's investigate the context overflow bug")).toBe(false);