fix: make selectedOptionBuilder required in ShadSelect widget of withSearch constructor

[muradab]

This pr adds a required parameter to ShadSelect.withSearch factory so that users will have to provided selectedOptionBuilder when using ShadSelect.

List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read and followed the Flutter Style Guide.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making.
• I followed the Data Driven Fixes where supported.
• All existing and new tests are passing.

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[nank1ro]

LGTM