fix(workbench): accept empty application-center text in WorkstationConfig

applicationCenterWelcomeMessage / applicationCenterDescription were declared
with default="" but a pattern requiring >=1 char (^[...]+$). Any tenant whose
stored workstation config left these fields empty or unset (the default) made
WorkstationConfig(**raw) raise a pydantic ValidationError, so GET
/api/v1/workstation/config returned 500 and the client rendered its full-screen
"system maintenance" overlay — /workspace/c/new appeared broken.

Relax both patterns from + to * so the empty string is accepted while keeping
the character whitelist for non-empty input. Add regression tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/backend/bisheng/api/v1/schemas.py

@@ -476,13 +476,13 @@ class WorkstationConfig(BaseModel):
     applicationCenterWelcomeMessage: str | None = Field(
         default="",
         max_length=1000,
-        pattern=r'^[\u4e00-\u9fff\w\s\.,;:!@#$%^&*()\-_=+\[\]{}|\\\'"<>/?`~·！￥（）【】、《》，。；：“”‘’？]+$',
+        pattern=r'^[\u4e00-\u9fff\w\s\.,;:!@#$%^&*()\-_=+\[\]{}|\\\'"<>/?`~·！￥（）【】、《》，。；：“”‘’？]*$',
         description="App Center Welcome Message",
     )
     applicationCenterDescription: str | None = Field(
         default="",
         max_length=1000,
-        pattern=r'^[\u4e00-\u9fff\w\s\.,;:!@#$%^&*()\-_=+\[\]{}|\\\'"<>/?`~·！￥（）【】、《》，。；：“”‘’？]+$',
+        pattern=r'^[\u4e00-\u9fff\w\s\.,;:!@#$%^&*()\-_=+\[\]{}|\\\'"<>/?`~·！￥（）【】、《》，。；：“”‘’？]*$',
         description="App Center Description",
     )
     recommendedApps: list[str] | None = Field(

src/backend/test/workstation/test_workstation_config_pattern.py

@@ -0,0 +1,51 @@
+"""Regression tests for WorkstationConfig application-center text fields.
+
+`applicationCenterWelcomeMessage` / `applicationCenterDescription` were added
+with `default=""` but a `pattern` that required at least one char (`...]+$`).
+Any tenant whose stored config left these empty or unset (i.e. the default)
+made `GET /api/v1/workstation/config` raise a pydantic ValidationError -> HTTP
+500 -> the client rendered its full-screen "system maintenance" overlay, so
+`/workspace/c/new` looked broken.
+
+The fix relaxes the pattern to allow the empty string (`...]*$`) while keeping
+the character whitelist for non-empty input.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from bisheng.api.v1.schemas import WorkstationConfig
+
+
+def test_missing_fields_fall_back_to_empty_default():
+    # Existing installs whose stored config predates these two fields:
+    # WorkstationConfig(**raw) must not raise when they are absent.
+    cfg = WorkstationConfig()
+    assert cfg.applicationCenterWelcomeMessage == ""
+    assert cfg.applicationCenterDescription == ""
+
+
+def test_explicit_empty_strings_are_valid():
+    # Admin cleared the text in 构建 -> 工作台; empty is a legal value.
+    cfg = WorkstationConfig(
+        applicationCenterWelcomeMessage="",
+        applicationCenterDescription="",
+    )
+    assert cfg.applicationCenterWelcomeMessage == ""
+    assert cfg.applicationCenterDescription == ""
+
+
+def test_normal_cn_en_content_is_valid():
+    cfg = WorkstationConfig(
+        applicationCenterWelcomeMessage="欢迎使用应用中心 Welcome!",
+        applicationCenterDescription="这里是描述, description.",
+    )
+    assert "欢迎" in cfg.applicationCenterWelcomeMessage
+    assert "description" in cfg.applicationCenterDescription
+
+
+def test_out_of_whitelist_char_still_rejected():
+    # The pattern is a character whitelist (injection guard); relaxing +->*
+    # must NOT weaken it for non-empty input.
+    with pytest.raises(ValidationError):
+        WorkstationConfig(applicationCenterWelcomeMessage="hi🚀")