Adds the algorithm-layer guarantee tests for
docs/KVC_ROUTER_ALGORITHM.md §4.1. The full Dispatch loop
lives in replay.py (HTTP + mooncake), but the policy-layer
guarantee is testable in isolation: under any reject
sequence, select() must keep returning a valid worker.
Cases:
- select returns a valid decision even after every (s,d)
is past τ_reject (degenerate fallback)
- |D|·τ_reject rejects suffice to explore every D
(cannot trap a session on one D under universal
rejection)
- degenerate fallback picks the least-rejected D
(Algorithm 1 line 4)
- per-(session, D) isolation: session A's blacklist
does not affect session B
- migration_reject_threshold=0 disables blacklist
- select() does NOT silently bump the reject counter
(the only mutator is record_admission_reject)
Adds tests/_fixtures.py with minimal make_topology() and
make_request() helpers that skip build_single_node_topology's
GPU-budget validation (irrelevant in unit tests).
Verified locally: 20/20 passing under pytest 9.0.3. The
six new tests cover only Algorithm 1's policy-layer
half of Theorem 1; the reset-on-success half lives in
Algorithm 3 (replay.py) and is a future test target.
