Fix: second sweep of Rails-idiom formatting bugs

[sorafujitani]

Re-opens #103 now that #102 has landed and its base branch was deleted. This PR now targets main directly; no content changes from #103.

Summary

Seven more formatter bugs surfaced by a systematic walk through Rails-flavoured Ruby patterns that weren't covered by #102. Three of them caused the formatter to emit code that no longer parses; the rest silently reshape user style.

Bugs fixed

ID	Severity	Symptom	Root cause
#4 / #5	Syntax break	begin/rescue/else/ensure/end → else...\nensure\nensure\n...\nend; begin/rescue/else/end → else...\nend\nend	Prism's ElseNode.location extends through the next clause keyword. The fallback rule's source-slice emission therefore re-prints whatever comes after — duplicating ensure or end.
#3	Syntax break	foo(<<~SQL, …) if cond\n …\nSQL → foo(…) if cond\n …\nSQL if cond — the if modifier sticks to the terminator line, so Ruby never closes the heredoc.	The bridge extends a heredoc-carrying statement's end_offset past the terminator, and the extended slice already contains the modifier text, but format_postfix still appended if cond after it.
#2	Comment corruption + non-idempotence	def demo(a, # c1\n b, # c2\n c) # c3 duplicated the first comment, ejected the rest into the body, and each format pass duplicated them again.	build_def_header rebuilds the signature from parameters_text, and later format_trailing_comment / format_leading_comments calls re-collect comments already embedded in the slice.
#9	Indent collapse	Chain continuations inside a brace-lambda / do-block (scope :x, -> { where(...)\n .where(...) }) were re-indented to the outer base_indent, erasing the nested scope.	reformat_chain_lines treats any .xxx line as chain continuation regardless of nesting.
#10	Blank-line insertion	if (sql = <<~SQL)\n …\nSQL\n execute(sql) gained a blank line between the terminator and execute.	The predicate's source slice retained the trailing \n past the heredoc terminator, which compounded with format_normal's own hardline.
#6	Style reshape	x = begin\n …\nend rewritten as x =\n begin\n …\nend.	format_variable_write unconditionally split block-valued assignments across two lines.
!@	Name rewrite	def !@ silently rewritten to def !.	extract_node_name returns Prism's canonical name (:!), discarding the @ suffix; the original slice lives on name_loc.

Before / after highlights

# #4: else + ensure no longer duplicates the keyword
begin
  risky
rescue => e
  :e
else
  :ok
ensure
  :final
end   # was: ensure\nensure...\nend\nend

# #3: heredoc + `if` modifier stays on the opener line
scope.push(<<~SQL, type: params[:type]) if params[:type]
  SELECT 1
SQL    # was: SQL if params[:type] — broke heredoc termination

# #2: multi-line def header preserves every inline comment exactly once
def demo(a, # first
         b, # second
         c) # third
  [a, b, c]
end

# #9: chain inside a brace-lambda keeps its local indent
scope :active_in, ->(period) {
  where(active: true)
    .where(last_seen_at: period)  # was: collapsed to col 4
}

# #6: inline `x = begin` stays inline
x = begin
  1
rescue
  2
end  # was: x =\n  begin\n    1\n  ...

# #10: no blank line after `if (… <<~SQL)` terminator
if (sql = <<~SQL)
  SELECT 1
SQL
  execute(sql)
end

# !@ preserves the @ suffix
def !@
  false
end

Changes

• ext/rfmt/src/format/rule.rs — reformat_chain_lines now skips lines that open a block ({, do, |).
• ext/rfmt/src/format/rules/begin.rs — format_begin_clause emitter handles ElseNode explicitly; used from both format_explicit_begin and format_implicit_begin_body.
• ext/rfmt/src/format/rules/body_end.rs — multi-line header detection with verbatim source emission + comment-mass-mark-emitted; shared push_body_and_end body/end emitter; line_end_offset helper.
• ext/rfmt/src/format/rules/if_unless.rs — format_postfix emits heredoc-containing statements verbatim; format_normal strips one trailing newline from the predicate slice; new statement_contains_heredoc_tail detector.
• ext/rfmt/src/format/rules/variable_write.rs — inline preservation when the block-valued RHS starts on the assignment's own line.
• lib/rfmt/prism_bridge.rb — DefNode name uses name_loc.slice to keep operator @ suffixes.
• spec/rails_idioms_round2_spec.rb — 9 round-trip + idempotence + Prism parse-check tests, one per bug.

Test plan

• cargo test --manifest-path ext/rfmt/Cargo.toml --lib — 127 passed
• bundle exec rspec — 128 passed (119 existing + 9 new)
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean
• bundle exec rubocop — clean
• All 25 round-2 synthetic Rails-idiom fixtures pass Prism parse after formatting, and are idempotent.

Supersedes #103 (closed when its base branch was deleted alongside the #102 merge).