feat: add minScrollableWidth option to Table stickyColumns

[ernst-dev]

Description

Adds an opt-in stickyColumns.minScrollableWidth option to the Table component so consumers can control the minimum scrollable space that must remain next to the sticky columns for the feature to stay active.

Background — this fills a gap in the existing feature, it does not add a new one. The Table already fully supports sticky first (and last) columns via stickyColumns={{ first, last }}. However, the feature silently deactivates whenever the remaining scrollable space drops below a hard-coded 148px (MINIMUM_SCROLLABLE_SPACE), and there is no way for a consumer to influence that threshold. On narrower tables this means a first column that the developer explicitly asked to pin simply stops being sticky, with no signal.

This PR exposes that threshold:

<Table
  stickyColumns={{ first: 1, minScrollableWidth: 0 }}  // keep the first column pinned even on narrow tables
  ...
/>

• New optional TableProps.StickyColumns.minScrollableWidth?: number (pixels).
• Threaded through useStickyColumns into the enablement check (isEnabled).
• Defaults to the existing MINIMUM_SCROLLABLE_SPACE (148) when omitted — fully backward compatible, no behavior change for existing consumers.

Files touched:

• src/table/interfaces.tsx — public StickyColumns.minScrollableWidth + doc comment.
• src/table/sticky-columns/interfaces.ts — internal StickyColumnsProps.minScrollableWidth.
• src/table/sticky-columns/use-sticky-columns.ts — plumbed into the store + isEnabled guard.
• src/table/internal.tsx — passes the prop from the public API into the hook.
• pages/table/sticky-columns-min-scrollable-width.page.tsx — new dev page.
• unit tests + documenter snapshot updated.

⚠️ Ticket mismatch note: This work was requested against SIM AWSUI-61614, but that ticket is actually "Feature request: Allow excess whitespace in warning text" (FormField/Alert) and is already auto-resolved — it is unrelated to Table sticky columns. Per the task instructions I implemented the described "Table sticky first column" feature and am flagging the discrepancy here. Since Table already ships sticky first columns, I filled the closest genuine gap (minScrollableWidth) rather than duplicating existing functionality.

Related links, issue #, if available: SIM AWSUI-61614 (see mismatch note above)

How has this been tested?

• New unit tests in src/table/sticky-columns/__tests__/use-sticky-columns.test.tsx:
  • minScrollableWidth=0 keeps sticky columns active when the default 148px threshold would deactivate them.
  • a large minScrollableWidth deactivates sticky columns even when the default threshold would allow them.
• Full Table unit suite green: 539 passed / 42 suites.
• npm run quick-build and full gulp build succeed; ESLint clean on all touched files.
• Manual verification via the new dev page (#/light/table/sticky-columns-min-scrollable-width), toggling minScrollableWidth on a narrow (600px) horizontally-scrollable table.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.61%. Comparing base (f97729a) to head (e49ba79).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4812   +/-   ##
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  Coverage   97.61%   97.61%           
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  Files         952      952           
  Lines       30816    30818    +2     
  Branches    11318    11320    +2     
=======================================
+ Hits        30081    30083    +2     
  Misses        688      688           
  Partials       47       47           

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