[WebParagraph] Fixing edge cases for wrapping text (with newlines) (#189858)

### Summary

Fixes newline (`\n`) handling and caret/selection geometry in
`WebParagraph` to match `SkParagraph` behavior, resolving misplaced
cursor issues in multiline text fields on the web.

Fixes https://github.com/flutter/flutter/issues/188320

---

### Background

In Flutter text editing (`RenderEditable`), caret positioning, line
navigation, and selection rely directly on `Paragraph` query APIs
(`getPositionForOffset`, `getBoxesForRange`, `getLineBoundary`,
`getLineNumberAt`, and `getGlyphInfoAt`).

Previously, `WebParagraph` diverged from `SkParagraph` in several key
newline edge cases:
1. **Physical vs. Logical Width:** Browser Canvas/DOM text measurement
treats `\n` as whitespace with positive advance width. In `SkParagraph`,
newlines occupy zero physical width while retaining their logical text
ranges.
2. **Trailing Newlines:** Text ending in a newline (e.g. `"Hello\n"`)
must generate an additional empty line where the cursor resides after
pressing Enter.
3. **Query Parity:** Hit-testing and boundary queries on lines ending
with or consisting entirely of newlines returned incorrect offsets or
collapsed bounds.

---

### Changes

- **Explicit Hard Line Break Tracking (`wrapper.dart`):**
- Separated hard line breaks (`\n`) from collapsible whitespace in
`_LineBuilder`.
- Added support for generating the trailing empty line when text
terminates with a hard line break.

- **Logical vs. Physical Text Ranges (`layout.dart`,
`paragraph.dart`):**
- Introduced `physicalTextRange` on `TextBlock` to exclude `\n` from
physical geometry calculations (bounds, selection rects, line widths)
while preserving `textRange` for logical text operations.
- Adjusted `getGlyphInfoAt` to report zero layout width for `\n` and
synthesize end-of-line glyph bounds for newlines outside visual blocks.
- Aligned `getPositionForOffset` and `getLineBoundary` with
`SkParagraph` hit-testing rules, correctly handling empty trailing lines
and cluster boundary affinity.
- Implemented `SkParagraph`-matching binary search in `getLineNumberAt`.

---

### Testing

- Added `test/webparagraph/paragraph_edge_cases_test.dart` (newline
wrapping and empty line creation).
- Added `test/webparagraph/paragraph_selection_caret_test.dart` (caret
hit-testing, affinities, and selection bounding boxes across multiline
text).
- Added `test/webparagraph/paragraph_line_metrics_test.dart` (line
geometry and width validation).
- Updated existing `webparagraph` unit tests to ensure 1:1 parity with
`SkParagraph`.

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Co-authored-by: Mouad Debbar <mouad.debbar@gmail.com>
Co-authored-by: Jackson Gardner <jacksongardner@google.com>