fix(rpc): catch :exit/:throw from erpc.call; handle {:exception, :terminating} (OPS-4536)

[palantir-valiot]

Summary

Catch :exit (including the {:exception, :terminating} form that erpc.call surfaces when the remote MFA does exit(R)) and :throw in Relix.RPC.execute/1. Previously these bypassed the rescue and propagated as unhandled exits, crashing callers such as TokenHelper.authorize_token_via_rpc (valiot_app) at the first occurrence.

The change:

• Wrap the :erpc.call in both rescue (for exceptions) and catch :exit, ... / catch :throw, ....
• Log the failure with module, function, arity, and node (same style as the existing rescue path).
• Surface a normalized {:error, {:rpc_exit, reason}} as resp_body so the RPC struct has status: :executed and RPC.response/1 returns {:error, ...} instead of crashing the process.
• Added TDD tests that deliberately cause the exact exit/throw shapes and assert clean {:error, ...} results.

Files changed: lib/relix/rpc/relix_rpc.ex, test/relix_test.exs, mix.exs, mix.lock, CHANGELOG.md.

Why

Root cause of OPS-4536 (eliot-lamosa-gto-prod): unhandled {:exception, :terminating} from erpc.call at lib/relix/rpc/relix_rpc.ex:60 (stack: Relix.RPC.execute/1 → TokenHelper.authorize_token_via_rpc → Context plug → GraphQL).

Triage: NOTIFY+FIX, severity high, category code_bug. This is a missing error-handling path on a first-party RPC helper; frequency 0 does not downgrade it.

Test plan

• mix test (was red for the new case before the edit; 7/7 green after: 1 doctest + 6 tests including two new ones for {:exception, :terminating} and throw)
• mix format
• Per baseline rules: ran mix hex.outdated, bumped allowed dev dep ex_doc 0.40.1 → 0.40.3 (and transitive makeup_erlang) in the same PR; updated mix.lock
• Version bumped to 0.1.5; one-line CHANGELOG entry added
• git diff inspected before commit (no debug prints, no scope creep, no empty diff)
• Always used git push-safe (never plain git push)
• TDD: failing test committed first, made green by the minimal targeted edit

Closes OPS-4536

[linear-code]

OPS-4536