refactor(parser): extract fill_heredoc_contents visitor helpers

[mpecan]

Summary

Refactors fill_heredoc_contents in src/parser/helpers.rs — the ~100-line recursive AST visitor with 12 arms (gated by #[allow(clippy::too_many_lines, clippy::match_same_arms)]) becomes a ~60-line dispatcher that delegates to per-category helpers:

• fill_each — generic walk over a &mut [Node] slice; absorbs every redirect loop AND Pipeline's command-walk.
• fill_command — Command (assignments + words + redirects).
• fill_if — If (condition + then + optional else + redirects). else_body: Option<&mut Node> via as_deref_mut().
• fill_body_and_redirects — Subshell / BraceGroup / For / Select (fused into one OR-pattern arm — all four have body + redirects).
• fill_cond_body_redirects — While / Until (condition + body + redirects).
• fill_case — Case (walk each pattern's optional body + redirects).

Both #[allow] attributes dropped; no remaining arms are textually identical and the function fits under the 60-line clippy cap.

Test coverage

Added 3 parable test cases in tests/parable/14_here_documents.tests to pin future regression prevention — these paths weren't directly exercised before:

• heredoc in if else branch — covers else_body.as_deref_mut().
• heredoc in case pattern body — covers fill_case's inner loop.
• heredoc nested while inside if else — covers multi-level recursion through the lexer's heredoc queue.

Test plan

• cargo fmt
• cargo clippy --all-targets -- -D warnings — no warnings
• cargo test — 252 passed (56 heredoc cases incl. 3 new ones)
• Oracle suite — 12 oracle_* tests pass (cargo test --test integration oracle_)
• AST output is byte-identical (constitutional: compatibility is correctness)

Stack

Part of the v0.2.0 refactoring cycle (#61). This is PR 7 of 10.

Closes #57

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