fix(html): preserve mailto: and tel: links (#2644)

[İlker Oğulcan SAYIN (iosayin)]

Resolves #2644

Summary of Changes

• Added mailto and tel to allowed schemes in _CustomMarkdownify.convert_a.
• Preserved their original URI strings without path manipulation.
• Added comprehensive unit test covering both mailto: and tel: link markdown conversions.

[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Community review, so it does not clear the merge gate. I checked out the branch and ran it.

What I ran (packages/markitdown, Python 3.13 venv, upstream main 4cc9fa1 as base):

• tests/test_html.py and tests/test_docx.py on this branch: 135 passed, 1 skipped.
• A small probe converting <a href="...">link</a> through MarkItDown().convert_stream(..., ".html") on main and on this branch.

The basic case works: mailto:team@example.com and tel:+1-555-0100 now come out as [link](...) where main drops them to bare text. javascript: and sms: still collapse to text.

The problem is the if scheme in ["mailto", "tel"]: pass branch. It skips the path escaping completely, and the escaping is what keeps the output a valid Markdown link. From the probe:

• tel:+1 555 0100 becomes [link](tel:+1 555 0100)
• mailto:a b@example.com becomes [link](mailto:a b@example.com)

I rendered both with markdown-it and they come out as literal text, not links. A phone number written with spaces is a plausible href. For comparison #2645 escapes the path and gives tel:+1%20555%200100, which renders as a link (the PR also adds a test for the Word hyperlink path).

Smaller notes:

• MAILTO: keeps its case on this branch ([link](MAILTO:team@example.com)), while #2645 normalizes it to mailto:. Harmless.
• The new test only covers two clean addresses, so it would not catch the space case. A parametrized case with a space in a tel: href would.
• A raw space in the query (?subject=Weekly report) also stays raw, as it does on #2645 and for http today.

Overlap: #2645 fixes the same thing. Suggest the maintainers pick one. I will leave a note there too.

I did not test other converters that use _CustomMarkdownify.