fix(ui5-flexible-column-layout): acc findings fixed

[PetyaMarkovaBogdanova]

2) FlexibleColumnLayout

• Fixed missing/empty accessible names on start arrow buttons in arrow-only layouts.
• Fixed ARIA usage for separators by omitting aria-valuenow when the grip is not shown.

Component	Issue	Not by spec	Good to have	Spec refs	Spec quote sentence	Coverage strength
FlexibleColumnLayout	Missing/empty button accessible names	Yes	No	ACC-253, ACC-264	"icon ... aria-label ... If text is missing (icon only button)"	Direct
FlexibleColumnLayout	Invalid aria-valuenow usage	Yes	No	ACC-264	"Omit the aria-valuenow attribute to indicate indeterminate progress"	Direct

Follow-up (not part of this PR):

• Nested interactive controls finding for separators.

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-13553--ui5-webcomponents-preview.netlify.app

[dobrinyonkov]

A few comments:

aria-valuenow change — LGTM. Omitting it when the grip isn't shown matches ACC-264 and aligns with the separator being non-focusable in that state.

Arrow tooltip={this.accStartSeparatorText} — only partial fix. accStartSeparatorText returns undefined when no accessibilityAttributes.startSeparator?.name is supplied AND showStartSeparatorGrip is false. That's exactly the arrow-only layout (arrowVisible: true, gripVisible: false in FCLLayout.ts), so the icon-only button still ends up nameless in that case — the original ACC-253 finding isn't fully resolved. Consider a dedicated accStartArrowText getter with its own i18n fallback (the current fallback string is "Resize between…", which is also semantically wrong for an arrow that switches layout rather than resizes).

PR description vs. diff. The body lists three findings (missing arrow names, nested interactives, invalid aria-valuenow), but the diff only addresses aria-valuenow cleanly and the arrow-name one partially. The "nested interactive controls" finding (Button inside role="separator" with tabindex) isn't touched. Could you either trim the description to match what's shipped, or follow up with the rest?

Tests. No coverage added. Would be good to lock in:

• aria-valuenow present iff grip is shown
• arrow button gets a tooltip attribute when a name is supplied