test: add coverage for metrics timing decorator (metrics.py)

[mnadzam]

Description

src/fromager/metrics.py has 52% test coverage but no dedicated tests. Existing coverage comes entirely from indirect execution -- tests that call decorated functions (e.g. download_source()) exercise the wrapper code but never validate timing storage or error behavior. The @timeit() decorator is applied across 3 modules with 8 total uses:

• build_environment.py (1 use)
• wheels.py (3 uses)
• sources.py (4 uses)

summarize() is called from 3 locations in bootstrap.py and build.py to report build performance.

A broken decorator could fail to propagate exceptions or corrupt timing data without any test catching it.

Scope

Function	Lines	Usage
timeit()	13-59	Decorator, 8 uses across 3 modules
summarize()	62-69	3 call sites
_extract_version_from_return()	72-80	1 (internal)

Acceptance Criteria

• timeit() -- test that timing data is stored in context after a successful call
• timeit() -- test that exceptions from the decorated function propagate correctly
• summarize() -- test log output format with accumulated timings
• _extract_version_from_return() -- test with iterable containing Version, bare Version, non-iterable, and None