…the edge of its own selection

## Description

Dragging text and dropping it on the end of its own selection deletes it. For example, in `Hello world`, select `Hello` and drop it right after itself: the paragraph becomes ` world`.

Root cause: `$doDrop` splits at the drop point first. When the range starts at the beginning of a text node, a drop on its end splits at the end of that same node, so `$splitTextPointCaret` returns a caret whose origin is the dragged node. `$isDropCaretInsideSelection` compares with `< 0` on both sides, so an edge drop counts as outside the range. `removeText()` then removes the dragged node, which is also the caret origin, and the detached origin check returns without inserting anything. A fully selected node (e.g. a bold word) dropped at the start of the next text node is lost the same way, because that edge resolves to the next node and the split rewinds the caret onto the dragged node.

Fix (the two changes suggested in the issue):

- `$isDropCaretInsideSelection` uses `<= 0`, so a drop exactly on either edge of the range is a no-op, like a drop inside it.
- If the caret origin was removed anyway (the edge resolved to the adjacent node), the data is inserted where `removeText()` left the selection, which is that same edge, instead of being dropped.

Neither change alone covers both cases.

Closes facebook#9232

## Test plan

Added a unit test to `HandleTextDrop.test.ts` that selects a bold `Hello` before ` world`, drops it at the end of `Hello`, then drops it at the start of ` world`, and checks the text is still `Hello world` after each drop.

### Before

```
$ npx vitest --project unit --no-watch packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
 × keeps the dragged text when dropped on the end edge of the source range
AssertionError: expected ' world' to be 'Hello world' // Object.is equality
      Tests  1 failed | 11 passed (12)
```

### After

```
$ npx vitest --project unit --no-watch packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
      Tests  12 passed (12)
```

The lexical-clipboard, lexical-rich-text and lexical-plain-text unit tests also pass (210 tests), as do `tsc -p tsconfig.test.json`, prettier and eslint on the changed files. The browser and e2e suites were not run locally.

## Description

Dropping a selection on an equivalent boundary in an adjacent node still
deletes and reinserts the selection in facebook#9241. In plain-text editors this
converts tokens and decorators into text. The detached-caret fallback also
inserts at the source when a cross-paragraph deletion removes the element
that held the destination.

Compare canonical text, element, and inline-wrapper boundaries before
mutating the document. Drops inside the selection or on either edge now
preserve node identity and selection direction. Normalize the destination
to text where possible, split only text, and retain both sides of the
insertion boundary so the destination survives paragraph merges. This also
avoids introducing paragraph breaks when an element point is hit.

Add 60 regression cases covering rich-text and plain-text handlers, forward
and backward selections, token interiors and edges, adjacent text and
element points, nested inline wrappers, decorators, and text/paragraph/root
destinations after cross-paragraph deletion.

This commit applies on top of PR facebook#9241 at
36d4dc2.

Closes facebook#9232

## Test plan

### Before

The new tests with clipboard.ts from 36d4dc2:

```
$ node_modules/.bin/vitest --project unit --run packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
 Test Files  1 failed (1)
      Tests  48 failed | 24 passed (72)
```

### After

```
$ node_modules/.bin/vitest --project unit --run packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
 Test Files  1 passed (1)
      Tests  72 passed (72)

$ node_modules/.bin/vitest --project unit --run packages/lexical-clipboard packages/lexical-rich-text packages/lexical-plain-text packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
 Test Files  26 passed (26)
      Tests  270 passed (270)

$ node_modules/.bin/tsc -p tsconfig.test.json --noEmit
$ node_modules/.bin/eslint packages/lexical-clipboard/src/clipboard.ts packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
$ node_modules/.bin/prettier --check packages/lexical-clipboard/src/clipboard.ts packages/lexical/src/__tests__/unit/HandleTextDrop.test.ts
Checking formatting...
All matched files use Prettier code style!
```

Browser/E2E tests were not run: Playwright's Chromium installation failed
because the browser archive download was invalid or truncated.