fix: TreeSelect selected value visible when searching (ant-design#56946)

* Initial plan

* Fix TreeSelect selected value visibility when searching

- Fixed CSS specificity issue where .ant-select-open .ant-select-content was overriding the transparent color
- Added nested rule for has-search-value within open state to ensure selected text is hidden during search
- Added test case to verify the fix

Co-authored-by: afc163 <507615+afc163@users.noreply.github.com>

* Add test for Select component search value hiding

- Added test case to ensure Select component also properly hides selected value when searching
- Verified fix works for both Select and TreeSelect components

Co-authored-by: afc163 <507615+afc163@users.noreply.github.com>

* Fix code formatting per Biome requirements

- Simplified JSX formatting in test files
- All linting checks pass
- Tests still passing

Co-authored-by: afc163 <507615+afc163@users.noreply.github.com>

* Remove test cases per feedback

- Removed "should hide selected value when searching" test from Select
- Removed "should hide selected value when searching" test from TreeSelect
- CSS fix in select-input.ts remains (the actual fix)

Co-authored-by: afc163 <507615+afc163@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: afc163 <507615+afc163@users.noreply.github.com>
Co-authored-by: afc163 <afc163@gmail.com>

Author: Copilot

components/select/style/select-input.ts

@@ -297,6 +297,10 @@ const genSelectInputStyle: GenerateStyle<SelectToken> = (token) => {
 
           [`&${componentCls}-open ${componentCls}-content`]: {
             color: token.colorTextPlaceholder,
+
+            '&-has-search-value': {
+              color: 'transparent',
+            },
           },
         },
       },