[Feature]: SearchBox - Support fixed/constrained width mode

[spenoff]

Area

React Components (@fluentui/react-components)

Describe the feature that you would like added

The v9 SearchBox does not support a fixed-width mode. In v8, the SearchBox had a default fixed width that could be controlled. In v9, the component always stretches to fill its container, requiring external CSS wrappers to constrain width. We currently have a workaround in place, but we think it is reasonable to expect Fluent UI to support features like this.

Use case: Multiple AMC pages (Profile display language, Privacy, Websearch) relied on the v8 SearchBox's fixed-width behavior. After migration to v9, all required additional wrapper elements with explicit width styles.

Proposed solution: Add a style or width prop, or expose a root slot that respects fixed-width configuration without requiring an outer wrapper div.

Additional context

• Previously filed as #33888 (by same author, different account) but was auto-closed by the inactivity bot. The issue was not resolved -- Miroslav Stastny offered a maxWidth: 'unset' style override workaround, but this is still just a workaround and not actual support in the component.
• Pages impacted: Profile display language, Privacy, Websearch
• This was supported in v8

Have you discussed this feature with our team

@miroslavstastny

Validations

• Check that there isn't already an issue that requests the same feature to avoid creating a duplicate.

Priority

Medium