[TE] Expand config env-var test coverage (#2409)

* [TE] Expand config env-var test coverage

Add tests for 7 env-var parsing patterns beyond MC_PKEY_INDEX:
- MC_NUM_CQ_PER_CTX: range [1,255], also sets JFC/JFCE
- MC_SLICE_SIZE: atoi to size_t, zero rejected
- MC_LOG_LEVEL: string enum (TRACE/INFO/WARNING/ERROR)
- MC_DISABLE_METACACHE: presence-based boolean
- MC_IB_TC: stoi with try-catch, range [0,255]
- MC_ENDPOINT_STORE_TYPE: string enum (FIFO/SIEVE)
- ValidatePortRange: 7 cases covering boundaries, ephemeral
  ports, well-known ports, min>max

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Strengthen sentinels, add MTU and port boundary tests

- Use non-default sentinel values (42, 99999) for rejection tests
  to distinguish "preserved" from "reset to default".
- Add valid MC_MTU tests (512/1024/2048/4096). Invalid MTU calls
  exit() so it is not tested here.
- Add ValidatePortRange boundary tests: first valid (1024), last
  before ephemeral (32767), above max (65536).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Add tests per review: negative slice_size, invalid log level, sentinel

- MC_SLICE_SIZE negative: documents existing wrapping behavior
- MC_LOG_LEVEL invalid: verifies unknown level keeps default
- MC_ENDPOINT_STORE_TYPE: use FIFO sentinel for rejection test

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* [TE] Reflect MC_MTU ignore-on-invalid semantics in tests

Per review: #2344 removed exit() from config parsing, so an invalid
MC_MTU is logged and ignored with the default kept. Fix the stale
comment and add an InvalidIsIgnored case asserting IBV_MTU_4096
survives MC_MTU=1500.

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>