feat(bash): add .ksh extension to bash parser (#235)

[azizur100389]

Summary

Register .ksh (Korn shell) with tree-sitter-bash alongside the existing .sh / .bash / .zsh entries shipped in #227. Korn shell is close enough to Bash syntactically that tree-sitter-bash handles the structural features the graph captures (function definitions, commands, source/. includes) correctly.

Closes #235.

Why this PR

In the close comment on #230 you explicitly flagged this as worth a follow-up:

If there's anything your implementation does that mine doesn't (e.g. .ksh extension, additional test coverage), please open a follow-up issue pointing at the specific feature and I'll merge it in. The .ksh extension in particular looks worth adding — I didn't include it in #227.

This PR is exactly that: the tracking issue (#235) plus the minimal change to address it.

Why it matters

Korn shell is still used in legacy AIX/Solaris operations, IBM internal tooling, and enterprise CI scripts. Repositories that ship .ksh scripts currently index to 0 nodes because the extension is unrecognized — the same failure mode that motivated #197.

Implementation

A single line added to EXTENSION_TO_LANGUAGE in code_review_graph/parser.py:

".ksh": "bash",  # Korn shell — close enough to bash for tree-sitter-bash (#235)

All of the bash parsing machinery shipped in #227 (_FUNCTION_TYPES, _CALL_TYPES, _extract_bash_source_command, name/call resolution) already handles any file routed through the "bash" language path — so no further wiring is needed.

Tests added (tests/test_multilang.py::TestBashParsing)

1. test_detects_language — extended with a .ksh assertion to lock the mapping in as a regression guard.
2. test_ksh_extension_parses_as_bash — end-to-end regression test that copies tests/fixtures/sample.sh to a temp legacy.ksh, parses it through CodeParser.parse_file(), and asserts:
   • Every node's language field is "bash"
   • The set of extracted Function names is identical to the .sh run
   • The CONTAINS / CALLS / IMPORTS_FROM edge counts per kind match

The second assertion specifically proves .ksh is fully wired through the same structural extraction path as .sh, not a degenerate zero-result read.

Test results

Stage	Result
Stage 1 — new targeted tests	2/2 passed
Stage 2 — tests/test_multilang.py full	152/152 passed — zero regressions
Stage 3 — adjacent tests/test_parser.py	67/67 passed
Stage 4 — full suite	733 passed, 8 pre-existing Windows failures (test_incremental x3 + test_main async detection x1 + test_notebook Databricks x4) — verified identical on unchanged main
Stage 5 — ruff check	clean on parser.py and test_multilang.py
Stage 6 — end-to-end smoke	detect_language("legacy.ksh") → "bash"; parsing a real .ksh file produces 6 Function nodes, 18 edges, all tagged language=bash

Zero regressions. Single-line extension mapping change plus a targeted regression guard.