fix(Select): announce result count with the active option for improvedA11y combobox

[shubham-kumar1-eightfold]

SUMMARY:

The improvedA11y combobox echoed the highlighted option into the result-count live region, overwriting the count — so screen readers stopped announcing "N matches found" while filtering (regression surfaced after #1149). This combines the count and option in one polite announcement instead.

What screen readers now hear (single live region):

• Open / filter: "{count} matches found. {option}, {group}, {n} of {total}" — one announcement, including the seeded option's group.
  • VoiceOver merges live-region updates into one utterance, so racing separate count/option regions dropped one of them.
  • VoiceOver doesn't read the seeded option's group natively on open, so we include it here.
• Arrow navigation: "{option}, {n} of {total}" — option only; no count (avoids re-announcing it as noise) and no group (VoiceOver reads the option + its group natively while arrowing, so echoing them would double-announce).
• Debounced on open/filter so per-keystroke updates coalesce into one announcement and don't race the combobox input's own typing echo; arrow-nav announces immediately.
• Plain (non-improvedA11y) Selects are completely unchanged — all of the above is gated behind improvedA11y.

Implementation (minimal delta on #1149):

• liveRegionMessage now appends the option to the count instead of overwriting it, gated by an announceModeRef ('list' on open/filter, 'nav' on arrow — set in the seed effect and moveActiveDescendant).
• Group comes from an optionId → group map memoized on options (O(1) lookup, no per-announcement rescan), also gated to improvedA11y.
• Debounce is a useEffect + setTimeout (the repo's standard pattern, e.g. Accordion), gated to improvedA11y; the delay is the ANNOUNCE_DEBOUNCE_DURATION constant in Select.types.ts, matching the repo's timing-constant convention (ANIMATION_DURATION, …).
• Adds one exported timing constant (ANNOUNCE_DEBOUNCE_DURATION), consistent with the existing ones (ANIMATION_DURATION, …); no new component props, no locale changes.

GITHUB ISSUE (Open Source Contributors)

N/A

JIRA TASK (Eightfold Employees Only):

https://eightfoldai.atlassian.net/browse/ENG-203165
Follow-up to the combobox VPAT work in #1149.

CHANGE TYPE:

• Bugfix Pull Request
• Feature Pull Request

TEST COVERAGE:

• Tests for this change already exist
• I have added unittests for this change

TEST PLAN:

• Updated Select.test.tsx: open announces the count and Apple, 1 of 3; after ArrowDown the region announces Banana, 2 of 3 without the count.
• Added a grouped test: open announces the seeded option's group (Apple, With HRIS); ArrowDown announces Banana, 2 of 2 without the group (left to VoiceOver's native reading).
• Full Select.test.tsx suite passes (81 tests); Form suite unaffected (the improvedA11y gate keeps plain Selects untouched).
• Verified with VoiceOver + Safari on grouped and ungrouped template Selects.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.00%. Comparing base (fe124ee) to head (f759a78).

Files with missing lines	Patch %	Lines
src/components/Select/Select.tsx	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1152      +/-   ##
==========================================
+ Coverage   84.98%   85.00%   +0.01%     
==========================================
  Files        1230     1230              
  Lines       21555    21588      +33     
  Branches     8193     8209      +16     
==========================================
+ Hits        18319    18351      +32     
- Misses       3150     3151       +1     
  Partials       86       86              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[agupta-eightfold]

lgtm