feat(llm): enforce terminal stream contract for every route

[rekram1-node]

Stacked 3/3 on #36692 (which stacks on #36691).

Summary

LLMClient.stream now guarantees, for native protocol routes and the synthetic AI SDK route alike:

• A successful stream emits exactly one terminal finish event and nothing after it.
• EOF before finish means the provider stream was truncated (proxy cut, silent drop) and fails as LLM.MalformedResponse instead of letting a partial response settle as complete.
• Output after finish fails the stream.

Enforcement applies after protocol parsing so stream.onHalt flushes remain subject to it, with per-subscription state.

Previously only generate detected missing terminals (via response assembly); stream consumers could observe silent truncation as a clean end.

Testing

• packages/llm: typecheck clean, 309 pass / 0 fail
• packages/core: typecheck clean, 1270 pass / 0 fail
• Final tree is byte-identical to the original combined branch from closed refactor(llm): redesign error model as flat tagged union #36622.

[kitlangton]

Heads up: #36881 just merged to v2 with essentially the same terminal guard, since this was biting production today (a truncated write tool-input stream settled as execution.succeeded, details in #36766).

Two deltas from this PR's version, both driven by it landing on current v2 rather than on the provider-error removal stack:

• The guard accepts finish or provider-error as terminal. Once refactor(llm): remove the provider-error stream event #36692 lands, that predicate collapses to finish-only.
• Enforcement lives inside streamPrepared in makeFromTransport rather than in streamRequestWith, so direct route.streamPrepared consumers are covered too, and it reuses the route label already computed there. There is also a provider-neutral regression in test/adapter.test.ts.

So this PR should rebase down to roughly the predicate simplification plus whatever the flat error union renames. Sorry for the churn, and happy to help with the rebase if useful.