[BUGFIX] Allow to clear unit selector input when typing

[adrianSepiol]

Fixes: perses/perses#4281

Description

Problem
The UnitSelector component used MUI Autocomplete's built-in clear button, which fires onChange with null when the user deletes all text from the input field. This caused the fallback logic to silently reset the unit to Decimal — even when the user was mid-edit and hadn't confirmed a new selection.

Fix

• Set disableClearable on the SettingsAutocomplete to prevent MUI from firing onChange(null) on text deletion.
• Introduced an explicit IconButton with a ClearIcon as a controlled clear action. This button only resets the value to undefined (Decimal) when the user deliberately clicks it.

Behavior Changes
Previously: deleting all text from the input and clicking away would reset the unit to Decimal.
Now: deleting all text and clicking away leaves the current value unchanged. Only clicking the ✕ button explicitly resets the unit to Decimal.

Screenshots

Screen.Recording.2026-08-18.at.11.49.52.mov

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.