Fix SelectCurrent update before #label is composed

[VedK5643]

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#6581

Summary

Fixes #6581.

This PR fixes a crash that can occur when SelectCurrent.update() is invoked before the widget has composed its #label child.

The crash occurs because query_one("#label") raises NoMatches during early widget initialization.

This change preserves the pending label state until composition and adds a regression test covering the issue.

Changes

• Handle NoMatches when #label is not yet available.
• Initialize the composed label using the stored label value.
• Add a regression test reproducing the issue.

Testing

• Added a regression test for the reported crash.
• Verified the test fails on the original implementation.
• Verified the test passes with the fix.
• Ran the existing Select test suite successfully.