[web][a11y] Initialize slider semantics values when enabling - #195564

When a Slider is enabled after its semantics node already exists, the web engine attaches a new SemanticIncrementable role to a node whose value, increasedValue, and decreasedValue fields are not dirty. _updateInputValues() then returns early on the role's first update(), so the hidden <input type="range"> is exposed to assistive technology without value, min, max, aria-valuetext, aria-valuenow, aria-valuemin, or aria-valuemax. The browser announces the default range value ("50") until the slider's value changes once.

This change starts _pendingResync as true, so a newly attached role performs one initial input-value sync on its first update() — the approach proposed by @cashmula in the issue's "Likely cause" analysis, which also identified the root cause. The flag is cleared after that sync; its existing role in change-event handling is unchanged. SemanticsNodeUpdate normalizes absent value fields to empty strings, so the initial sync cannot hit null-asserts.

Three regression tests are added to test/engine/semantics/semantics_test.dart: enabling a previously disabled slider with unchanged values (the reported bug), disabling and re-enabling a slider (role reattachment resync), and role attachment while the engine is in pointer-events gesture mode (input stays disabled but attributes still sync).

The first test fails on unmodified code: the input renders as bare <input type="range" role="slider" tabindex="0"> with value="50".

Verified locally with felt on Windows against --gcs-prod artifacts matching the base revision:

• chrome-dart2js-canvaskit-engine, all 12 test/engine/semantics files: 280/280 tests passed.
• chrome-dart2wasm-canvaskit-engine, semantics_test.dart: all 191 tests passed.
• dart analyze and dart format clean on the changed files.

Limitations: tested in Chrome with the CanvasKit renderer only (the engine test-set has no skwasm suite in felt_config.yaml); Skia Gold screenshot checks were skipped locally; verified at the engine DOM level, not with a live screen reader. Upstream CI has not run yet.

AI disclosure

The patch implementation and tests were prepared with assistance from Devin AI under human direction. I have independently reviewed the complete diff and understand the changes.

Fixes #195215

No changes to the flutter/tests repo were needed, so no migration guide applies.

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