Closes https://github.com/zed-industries/zed/issues/59083
Closes https://github.com/zed-industries/zed/issues/42958
Closes https://github.com/zed-industries/zed/issues/59612

This PR improve selection copy of Agent Panel and Markdown Preview such
that copied text is always valid markdown, following how VSCode handles
it. The partial selection of styled text would not copy broken syntax
like `bold**` or ```inline code` ``` like before.

Now we follow simple rule:

> Selecting any part, no matter from where, copies it as its markdown.
Except when the selection sits entirely inside a single inline code
span, in which case we copy plain text, for terminal and code use cases.

Examples:

This is **bold** text, this is *italic* text, and this is `code` all `in
one` sentence.

- selecting only bold → `**bold**`
- selecting normal text and partial bold → `is is **bo**`
- selecting a single code span completely → `code`
- selecting partial code → `od`
- selecting partial text and code → `` his is `cod` ``
- selecting multiple code spans partially → `` `ode` all `in o` ``
- selecting multiple code spans end to end → `` `code` all `in one` ``

Nested spans, like **bold with `code` inside**:

- partial text in bold → `**ld wi**`
- partial code in bold → ``**`od`**``
- full code in bold → ``**`code`**``
- the whole sentence → ``**bold with `code` inside**``

Links, like [Visit Rust's website](https://rust.org):

- partial link text → `[bsite](https://rust.org)`
- full link text → `[Visit Rust's website](https://rust.org)`

How it works:

Selection boundaries that land inside delimiter syntax (`**`, backticks,
etc.) first snap out so no delimiter is left half-selected. Then any
spans the selection cuts through get their delimiters re-added,
outermost first, so nested styling stays balanced. Only the root blocks
containing the two selection boundaries are inspected, everything in
between is copied right as is, which also keeps this cheap on large
documents.

Release Notes:

- Improved copying selected text in Agent Panel and Markdown Preview.
Partial selections of styled text now copy as well-formed markdown, and
selections within a single inline code span copy as plain text.