[Aditya-Saroha]

In raising this pull request, I confirm the following (please check boxes):

Reason for this PR:

• This PR adds new functionality.
• This PR fixes a bug that I have personally experienced or that a real user has reported and for which a sample exists.
• This PR is porting code from C to Rust.

Sanity check:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• If the PR adds new functionality, I've added it to the changelog. If it's just a bug fix, I have NOT added it to the changelog.
• I am NOT adding new C code unless it's to fix an existing, reproducible bug.

Repro instructions:

Sample: markup.bin attached to issue #2437 (a 77-byte RCWT sample containing <BOB> & HI).

Commands to reproduce:

ccextractor markup.bin --out=webvtt -o markup.vtt
ccextractor markup.bin --out=smptett -o markup.xml
python3 -c 'import xml.etree.ElementTree as E; E.parse("markup.xml")'

Before fix:

• WebVTT writes & HI unescaped. When parsed in a browser (e.g., Chromium cue.getCueAsHTML().textContent), is parsed as an unknown HTML element and omitted from text display, leaving only & HI.
• SMPTE-TT writes & HI.... XML parsing fails with ParseError: not well-formed (invalid token): line 20, column 21 due to the unclosed tag and bare &.

After fix:

• WebVTT escapes literal caption characters to & HI, preserving & HI as visible text in browser cues.
• SMPTE-TT escapes literal characters in get_decoder_line_encoded(), producing valid XML that parses cleanly with xml.etree.ElementTree.
• Formats without HTML/XML entity requirements (such as SRT, SSA, SAMI, SPUPNG) remain untouched, preservingraw text as decided in [FIX] escape special characters ( < > & ) to HTML context #1703.

---

Description of Changes

Fixes #2437.

1. src/lib_ccx/ccx_encoders_webvtt.c:

• In write_cc_buffer_as_webvtt(), escape literal caption characters (& -> &, < -> <, > -> >). Encoder-generated styling tags (, , <c.color>, , ) are written separately and remain unescaped.

2. src/lib_ccx/ccx_encoders_helpers.c:

• In get_decoder_line_encoded(), escape literal caption characters when ctx->write_format == CCX_OF_SMPTETT. Encoder formatting tags (, , ) are inserted separately and remain untouched, allowing write_cc_buffer_as_smptett() to convert them to TTML style tags while producing valid XML text.

3. src/lib_ccx/ccx_encoders_smptett.c:

• In write_cc_buffer_as_smptett(), updated strchr calls searching for # and > in font-colored blocks to search starting from start rather than context->subline, avoiding premature matches if text preceding the tag contains # or >.

Verification

• Verified SMPTE-TT XML parses cleanly via python3 -c "import xml.etree.ElementTree as E; E.parse('...')" with exit code 0 and text & HI.
• Verified WebVTT cue payload decodes properly in Chromium (VTTCue.getCueAsHTML().textContent gives & HI).
• Verified styled captions (, , color tags) retain formatting elements while escaping inner special characters.
• Verified SRT output remains unescaped (regression check).