refactor(infer): consolidate args.rs text-scan helpers onto CST path

[Hessesian]

Status (re-audited 2026-06-17)

The original premise is partly outdated. A CST fast path (cst_call_arg_type) was already added to find_as_call_arg_type in #24 (2026-04-29, a month before this issue was filed), so the text-scan helpers are already fallbacks that only run when no live tree is available — not the primary path.

There are now four independent enclosing-call detectors that don't share a base:

• cst_call_info (cst_cursor.rs) — authoritative; used by signature help & traits_impl
• cst_call_arg_type (args.rs:391) — a separate inline CST walk; used only by find_as_call_arg_type
• find_enclosing_call_name (scope.rs:695) — text scan; 3 callers (args, receiver, scope)
• find_call_context (args.rs:122) — text scan; 1 caller (find_positional_call_arg_type)

So the real duplication is broader than originally described, and it now includes a third CST variant — not just the two text helpers.

Rescoped goal — one CST-first detector + one text fallback

• Fold cst_call_arg_type into cst_call_info (or have args.rs call cst_call_info directly) so there is a single CST call/arg detector.
• Delete find_call_context; fold its depth-aware comma counting into find_enclosing_call_name as the single text fallback.
• Converge find_named_call_arg_type / find_positional_call_arg_type onto that CST-first-with-text-fallback path.

Priority: low

The text helpers are fallback-only (no-live-tree case, rare for the file being edited) and are not directly unit-tested — only the entry point find_as_call_arg_type is (with a dedicated CST-fast-path test section). Pure cleanup: modest benefit, non-trivial regression risk.

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Original problem (retained for context)

src/indexer/infer/args.rs has two independent text-scan implementations for detecting the enclosing call and argument position:

• find_named_call_arg_type() (lines 69–107): calls find_enclosing_call_name() from scope.rs
• find_positional_call_arg_type() (lines 109–119): uses its own find_call_context() helper (lines 122–196) with separate depth-aware comma counting

Both duplicate logic that cst_call_info() in src/indexer/infer/cst_cursor.rs already handles correctly with a CST-first + text fallback strategy.

Goal

• Converge both functions to use cst_call_info when a live tree is available
• Remove or merge find_call_context with find_enclosing_call_name in scope.rs
• Single source of truth for enclosing-call detection

Discovered by

Audit of codebase after Issue #142 (CompletionContext centralisation).