Add IRB.doc_providers, a hook for external documentation providers

[znz]

Implements the proposal in #1243: a small public extension point so that documentation backends other than RI can plug into show_doc and the documentation dialog of the autocompletion (Alt+d) without monkey-patching irb internals.

API

IRB.doc_providers is the ordered list of providers consulted by show_doc NAME and by the completion dialog. It defaults to [IRB::RDocDocumentProvider.new], so nothing changes out of the box. A library or .irbrc can add its own provider; earlier providers win:

IRB.doc_providers.unshift(MyDocProvider.new)

A provider is a duck type:

class MyDocProvider
  # name is RI-style: "Array", "Array#each", "Array.new", "String.gsub"
  # (the completion uses a dot even for instance methods).
  # Return a String (shown through IRB::Pager, ANSI allowed) or nil to let
  # the next provider answer.
  def document(name) end

  # Optional: lines for the completion dialog that fit in `width` columns, or nil.
  def dialog_contents(name, width) end
end

IRB::RDocDocumentProvider is the existing RI code moved behind this interface (driver construction honoring IRB.conf[:EXTRA_DOC_DIRS], document, dialog_contents, interactive, and not_found_message, which reproduces the "maybe you meant" suggestions of ri). show_doc without an argument still starts RI's interactive session directly; it is RI-specific and does not consult providers. This is documented in doc/EXTEND_IRB.md.

Changes

• lib/irb/doc_provider.rb (new): IRB.doc_providers and IRB::RDocDocumentProvider.
• lib/irb/command/show_doc.rb: asks the providers in order and pages the first result with IRB::Pager. When nothing is found it prints RI's suggestions ("X not found, maybe you meant:" / "Nothing known about X"), or the existing "Can't display document because rdoc is not installed." warning when RDoc is missing.
• lib/irb/input-method.rb: the dialog (doc_dialog_contents) and the Alt+d full document (display_document) go through the providers. The dialog is registered when any provider can serve it, so a non-RDoc provider also works without rdoc. The error fallback from Keep completion alive when RDoc document retrieval fails #1229 is kept (a raising provider shows the error in the dialog instead of breaking completion) and no longer needs RDoc to render it.
• Tests: test/irb/test_doc_provider.rb (new), plus provider-based tests in test/irb/test_input_method.rb and test/irb/test_command.rb using stub providers, so the dialog and show_doc code paths are now exercised even on CI without RI data. with_doc_providers helper in test/irb/helper.rb.

Behavior notes

Mostly a refactor, with these visible differences:

• show_doc NAME output is paged by IRB::Pager (honoring IRB.conf[:USE_PAGER]) instead of RI's own pager, and uses ANSI bold on a color terminal (like the dialog already did). When stdout is not a tty the text is the same backspace-bold text as before.
• Names are resolved with RI's expand_name like the ri command does, so show_doc StringI shows StringIO and show_doc Strin lists similar classes (both printed nothing before). Unknown names are still reported as "Nothing known about NAME".
• The "rdoc is not installed" warning is only printed when no provider could answer, and Ctrl-C in the interactive RI session (show_doc without arguments) no longer prints that warning by mistake.

With this in place, bitclust-irb (rurema/bitclust#326) can register a provider instead of a separate refe command, so show_doc String#gsub shows the Japanese manual when available and falls back to RI.

日本語要約

#1243 の提案の実装です。IRB.doc_providers(既定は [IRB::RDocDocumentProvider.new])に登録したプロバイダを show_doc と補完のドキュメントダイアログ(Alt+d)が順に問い合わせます。プロバイダは document(name)(必須・String か nil)と dialog_contents(name, width)(任意)を持つ duck type です。既存の RI 実装は IRB::RDocDocumentProvider に移動し、show_doc 引数なしの対話モードは従来どおり RI 直結です。

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