html/template: handle CDATA in foreign script elements

[carrerasdarren-cell]

HTML tokenizers recognize CDATA sections when the current node is SVG or
MathML. An apparent closing script tag inside such a section is text, but
html/template currently ends its script context there. A later action is
therefore HTML-escaped instead of JavaScript-escaped. In a template literal,
attacker-controlled interpolation can execute.

This change tracks the foreign namespace transitions that affect CDATA,
including HTML integration points and breakout tags, and preserves the
JavaScript context until the CDATA section ends. Dynamically ambiguous
attributes retain the stricter JavaScript-escaping direction.

The issue reproducer was confirmed on current master. With an inert local
payload that assigned 1337 to a sentinel, headless Chrome changed the sentinel
from 0 to 1337. The patched rendering escaped the interpolation syntax and the
sentinel remained 0.

Validation included the complete short standard-library suite, ten
race-enabled html/template runs, 100 ordinary html/template runs, and go vet.

Fixes #62617.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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