test: add unit tests for `languages/` base, `benchmarking/`, `picklepatch/`

[KRRT7]

Parent: #2079

Phase 2 — Low coverage modules (38–45%)

Module	Stmts	Current coverage
languages/ (base)	1,830	38%
benchmarking/	1,518	43%
picklepatch/	163	45%

languages/ base

Key files:

• languages/function_optimizer.py (1,324 stmts, 21%) — the core per-function optimization loop
• languages/base.py (182 stmts, 94%) — nearly done
• languages/registry.py (145 stmts, 72%)

benchmarking/

Key files:

• benchmarking/compare.py (700 stmts, 37%) — performance comparison logic
• benchmarking/plugin/plugin.py (274 stmts, 26%) — pytest benchmarking plugin
• benchmarking/codeflash_trace.py (105 stmts, 14%)
• benchmarking/utils.py (65 stmts, 45%)

picklepatch/

• picklepatch/pickle_patcher.py (134 stmts, 49%)
• picklepatch/pickle_placeholder.py (29 stmts, 28%)

Target

100% coverage for these modules (~3,511 statements).