text: Keep document image sources URI-backed

[lurenjia534]

Closes #2733

Description

Keep image sources originating from TextView documents URI-backed instead of
implicitly converting file:// and scheme-less strings into Resource::Path.

The previous conversion allowed Markdown and HTML content, including LSP hover
and completion documentation, to trigger reads of attacker-selected local
filesystem paths.

Inline images, block images, and intrinsic-size measurement already share the
image_source helper, so this change is confined to that conversion and its
existing unit test. Relative paths, absolute paths, Windows paths, and
file:// values are now all kept as Resource::Uri.

This intentionally removes implicit local-filesystem image loading from generic
TextView content. As a result, relative images such as
website/public/logo.svg in the Story Gallery README will no longer load as
local files. Supporting trusted local document resources requires a separate,
explicit API; base-directory confinement, path normalization, file-size
limits, and other resource-policy changes are outside the scope of this patch.

How to Test

The following checks pass:

cargo test -p gpui-component text::utils::tests::test_image_source -- --nocapture
cargo fmt --all -- --check
cargo test -p gpui-component

The updated test_image_source verifies that remote URLs, data URLs, relative
paths (including .. traversal), Unix absolute paths, Windows paths, paths
containing colons, and file:// values all remain Resource::Uri.

Manual runtime verification was also performed on Linux with the existing
markdown_table example and a Markdown document containing:

![poc](/tmp/textview-local-read-poc.svg)

Under strace, the vulnerable implementation opened the referenced SVG:

openat(AT_FDCWD, "/tmp/textview-local-read-poc.svg", O_RDONLY|O_CLOEXEC) = 28

With this patch, the example opened the injected Markdown document but did not
issue any open or openat call for the referenced SVG.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI-generated code is accurate.
• Passed cargo run for story tests related to the changes. The Story
  Gallery starts successfully; the README relative image is no longer loaded
  from the local filesystem, as documented above.
• This change is not platform-specific.

[huacnlee]

cc @madcodelife