chore: port v9.15.x patches into the v10 migration snapshot

[ze-flo]

Summary

Forward-ports the substantive v9.15.1–v9.15.8 fixes from the v9 branch into .packages/ — the frozen v9.15.0 snapshot that serves as the migration source for v10 single-package development — and prepends the corresponding entries to CHANGELOG.md.

Cherry-picked fixes:

• #2115 fix(dropdowns): avoid unwanted page scroll when Combobox mounts or re-renders
• #2123 fix(accordions): prevent focus ring clipping in stepper content
• #2124 fix(dropdowns): guard Listbox and MenuList state setters against post-unmount updates
• #2126 fix(dropdowns): restore isMountedRef on remount to fix StrictMode regression
• #2127 fix(modals): upgrade container-modal to v2 to fix blur-close behavior
• #2130 fix(modals): keep tooltip dialog within viewport bounds
• #2137 fix(accordions): prevent step content overflow when collapsed
• #2140 fix(dropdowns): stop auto-activating first option while editing autocomplete Combobox
• #2143 fix: keep components behavior stable for users on styled-components >= 6.3.12

Intentionally excluded:

• chore(release): publish version bumps (v9-specific; main uses pnpm + v10.0.0-next versioning)
• #2116 chore(ci): v9-branch CI configuration
• #2125 came through empty — main's lerna.json already reflects it

Notes:

• No src/ changes: the v10 accordion has no stepper yet (so #2123/#2137 can't apply), and src/ has zero .attrs usage (so #2143 has nothing to fix there). Fixes will ride into src/ as each component migrates.
• Conflict resolutions: kept main's package.json field ordering (applied only the container-combobox ^2.0.9 bump), accepted package-lock.json deletions, and merged import blocks in 4 spec files.

Test plan

• CI passes (.packages/ is not covered by the v10 build/test/lint configs; changes are snapshot-only)
• Spot-check cherry-picked diffs against their v9 sources
• Verify CHANGELOG.md renders correctly above the v10.0.0-next entries

[coveralls]

Coverage is 88.199% — port/v9.15.x-to-main into main. No base build found for main.