fix(py-lsp): memoize + guard expression evaluation (exponential re-eval hang)

py_eval_expr_type evaluated a call node's attribute receiver twice (once
in the container special-case, once in the general attribute path), so an
N-link chained call cost O(2^N) evaluations — real-world ~65-link builder
chains hung indexing for hours (DeusData#710). The evaluator also recursed once
per expression nesting level with no depth guard, so a pathologically deep
expression overflowed the native stack (DeusData#720).

Three guard layers, mirroring c_eval_expr_type's design in c_lsp.c:

1. Memoization: py_eval_expr_type is now a wrapper around the renamed
   py_eval_expr_type_uncached. Results are cached per node in an
   open-addressing table on PyLSPContext (arena-allocated, cold per file),
   keyed by node identity (TSNode.id) — NOT the start byte, since every
   leftmost descendant of a chain shares it and byte-keyed entries alias
   distinct nodes into silently wrong resolutions. Entries carry a scope
   generation; every scope bind/restore bumps it (O(1) whole-cache flush),
   so re-evaluation under changed bindings (lambda re-walks, isinstance
   narrowing) behaves exactly as before. Inserts are load-bounded (<75%)
   and rejected when growth fails, so a full table can never spin.

2. Depth cap: PY_LSP_MAX_EVAL_DEPTH 256 (same as C_EVAL_DEPTH_LIMIT);
   past the cap the evaluator returns unknown instead of overflowing the
   stack. Truncated results — including any ancestor of a truncation —
   are never cached, so a depth- or budget-limited `unknown` cannot
   poison post-truncation reuse.

3. Step budget: PY_EVAL_MAX_STEPS_PER_FILE 10000 (mirrors
   C_EVAL_MAX_STEPS_PER_FILE); pathological files degrade to unknown
   instead of stalling indexing. Cache hits don't consume budget.

Distills PR DeusData#732 (memoization; fixed the start-byte cache key and the
missing staleness handling) and PR DeusData#758 (depth cap; raised 64 -> 256 to
match the sibling LSPs, added the never-cache-truncated ordering), and
adds the step budget plus reproduce-first tests: a 40-link chain that
must resolve its final link, a heterogeneous-receiver chain that fails
under byte keying, a 30k-deep expression crash guard, and a budget
degradation guard.

Fixes DeusData#710. Fixes DeusData#720.

Co-authored-by: LA-10 <lashuha@uwaterloo.com>
Co-authored-by: Dustin Persek <dustin.persek@protonmail.com>
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

Author: 3 people