Make before_agent_start override merging preserve the first defined
model/provider override so higher-priority hooks cannot be overwritten by
lower-priority handlers, and align the corresponding test title and
expectation with the intended precedence behavior.
Co-authored-by: Cursor <cursoragent@cursor.com>
Layer 1: Hook merger tests verify modelOverride/providerOverride are
correctly propagated through the before_agent_start merger with
priority ordering, backward compatibility, and field isolation.
Layer 2: Pipeline wiring tests verify the earlyHookResult passthrough
contract between run.ts and attempt.ts, graceful error degradation,
and that overrides correctly modify provider/model variables.
19 tests total across 2 test files.
