fix(dub): name the source language code that was rejected

Closes #1960.

The report was "400 Bad Request: Invalid source language code" and nothing
else. That cannot be acted on or triaged: it does not say which of the ninety
or so codes was wrong, so neither the user nor a maintainer reading the
auto-filed issue can tell whether the picker offered something the backend does
not accept, or a stale preference from an older build is still being sent.

I could not determine the cause from the report, which is exactly the problem.
Naming the code makes the next one answerable instead of guessing at this one.

The value is a language code chosen from a menu, not private data, and the
engine validator a few lines away already echoes its input the same way.

Also adds the check I actually wanted while investigating: a test that reads
the picker's own LANG_CODES and asserts the backend accepts every one of them,
so a code added to the menu cannot silently become a 400. It passes today —
the menu and the allowlist do agree — which is how I ruled that out as the
cause rather than assuming it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017ypcgSsh5j2PEonSJiAU1S

CHANGELOG.md

@@ -9,6 +9,7 @@ the frozen-backend fallback mirror it for their toolchains.
 ## [Unreleased]
 
 **Highlights**
+- A rejected dubbing source language now names the code it rejected (#1960)
 - `bun run desktop` reclaims port 3900 from a backend the app itself left running, instead of refusing to start (#1974)
 - A dictation shortcut another app already owns now says so, instead of silently doing nothing (#1858)
 - Quitting on Windows is no longer reported as a crash on the next launch (#1898)

backend/api/routers/dub_core.py

@@ -540,12 +540,30 @@ _DUB_SOURCE_LANG_CODES = frozenset({
 
 
 def _source_lang_override(value: str | None) -> str | None:
-    """Normalize a user-selected source language; auto/und means detect."""
+    """Normalize a user-selected source language; auto/und means detect.
+
+    A rejection NAMES the code it rejected. "Invalid source language code" on
+    its own cannot be acted on or reported usefully: it does not say which of
+    the ninety-odd codes was wrong, so neither the user nor a maintainer
+    reading the auto-filed issue can tell whether the picker offered something
+    the backend does not accept, or a stale preference from an older build is
+    still being sent (#1960).
+
+    The value is a language code the user chose from a menu — not private
+    data — and the neighbouring engine validator already echoes its input the
+    same way.
+    """
     code = (value or "").strip().lower()
     if code in {"", "auto", "und"}:
         return None
     if code not in _DUB_SOURCE_LANG_CODES:
-        raise HTTPException(status_code=400, detail="Invalid source language code")
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"Invalid source language code: {code!r}. Pick a language from "
+                "the Dubbing source-language menu, or leave it on auto-detect."
+            ),
+        )
     return code
 
 

tests/test_dub_source_lang_1960.py

@@ -0,0 +1,68 @@
+"""#1960 — a rejected source language must say WHICH code it rejected.
+
+The report was "400 Bad Request: Invalid source language code" and nothing
+else. That cannot be acted on or triaged: it does not say which of the ninety
+or so codes was wrong, so neither the user nor a maintainer reading the
+auto-filed issue can tell whether the picker offered something the backend
+does not accept, or a stale preference from an older build is still being
+sent. I could not determine the cause from the report — which is the point.
+
+The value is a language code chosen from a menu, not private data, and the
+neighbouring engine validator already echoes its input the same way.
+
+The last test is the durable one: it reads the picker's own list and asserts
+the backend accepts all of it, so a code added to the menu cannot silently
+become a 400.
+"""
+import pathlib
+import re
+
+import pytest
+from fastapi import HTTPException
+
+from api.routers.dub_core import _DUB_SOURCE_LANG_CODES, _source_lang_override
+
+_REPO = pathlib.Path(__file__).resolve().parents[1]
+_LANGUAGES_JS = _REPO / "frontend" / "src" / "utils" / "languages.js"
+
+
+def test_the_rejection_names_the_code():
+    with pytest.raises(HTTPException) as caught:
+        _source_lang_override("zz-XX")
+    detail = caught.value.detail
+    assert "zz-xx" in detail
+    assert caught.value.status_code == 400
+
+
+def test_the_rejection_says_what_to_do():
+    with pytest.raises(HTTPException) as caught:
+        _source_lang_override("nope")
+    assert "auto-detect" in caught.value.detail
+
+
+@pytest.mark.parametrize("code", ["", "   ", "auto", "und", None])
+def test_detect_is_still_detect(code):
+    assert _source_lang_override(code) is None
+
+
+def test_accepted_codes_are_unchanged():
+    # This only improved a message; it must not start accepting or rejecting
+    # anything different.
+    for code in sorted(_DUB_SOURCE_LANG_CODES):
+        assert _source_lang_override(code) == code
+
+
+def test_every_code_the_picker_offers_is_accepted():
+    src = _LANGUAGES_JS.read_text(encoding="utf-8")
+    codes = re.findall(r"code:\s*['\"]([A-Za-z-]+)['\"]", src)
+    assert len(codes) > 50, "LANG_CODES did not parse; update this extraction"
+    rejected = []
+    for code in codes:
+        try:
+            _source_lang_override(code)
+        except HTTPException:
+            rejected.append(code)
+    assert rejected == [], (
+        "the Dubbing source-language menu offers codes the backend rejects, so "
+        f"picking them returns 400: {rejected}"
+    )