Avoid detecting encoding comments in documentation text

[st0012]

RDoc currently reads file content with RDoc::Encoding.read_file during RDoc::RDoc#parse_file, before parser selection. The current header regexp starts with ^ and permits arbitrary comment prose before coding:, so documentation text can be interpreted as an encoding directive before the selected parser handles the file (current regexp).

Ruby documents ^ as the beginning of a line and \A as the beginning of a string, and documents magic comments as top-level directives with plain encoding / coding syntax or Emacs-style -*- ... -*- syntax (Regexp anchors, magic comments). This PR updates RDoc header detection to stay at the file header and limits the coding: branch to those magic-comment forms (updated regexp).

The regression test covers documentation text that mentions #encoding: Returns, Encoding::US_ASCII.name, and a later prose # coding: line so those examples are treated as documentation text instead of encoding declarations (test fixture).

[Copilot]

Pull request overview

This PR tightens RDoc::Encoding’s header parsing so encoding “magic comments” are only detected at the start of a file and only in recognized magic-comment forms, preventing documentation prose from being misinterpreted as an encoding directive during RDoc::RDoc#parse_file.

Changes:

• Anchor header detection to the beginning of the string (\A) to avoid matching encoding directives mid-file.
• Restrict the coding:/encoding: branch to magic-comment forms (including Emacs -*- ... -*- style) rather than arbitrary comment prose containing coding: later in the line.
• Add a regression test ensuring documentation text mentioning #encoding: / Encoding::... / later # coding: is not treated as an encoding declaration.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/rdoc/encoding.rb	Refines HEADER_REGEXP to match encoding directives only in the file header and only in magic-comment formats.
test/rdoc/rdoc_encoding_test.rb	Adds a regression test for documentation-like comment text that previously could be misdetected as an encoding directive.

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[kou]

+1