Rename RestartExecution/s to ClaimFunction/s and add owner-guarded SetFunction

[stidsborg]

What & why

Groundwork that renames the store-level "restart" primitives to what they actually do — claim (take over ownership of a flow and return its current state) — and adds an owner-guarded state-setter. This unblocks a follow-up simplification of TryInlinePendingMessages (inlining a completed flow's late messages), which becomes: ClaimFunction → merge messages into the effect snapshot → SetFunction(status, owner: null, effects: merged, result: claimed.Result, expectedReplica: self, storageSession: claimed.StorageSession) → clear rows — a single owner-guarded write with no merge-write-verify retry loop.

Changes

• Rename IFunctionStore.RestartExecution → ClaimFunction, RestartExecutions → ClaimFunctions across the interface, all four stores + their SqlGenerators, test doubles, call sites (incl. PostponedWatchdog) and store test templates.
  • Kept unchanged (names only; store calls repointed): FlowsManager.RestartExecutions(IEnumerable<StoredMessages>) (message-delivery orchestration), InvocationHelper.RestartFunction, and the RestartedFunction record.
• Add IFunctionStore.SetFunction — an owner-guarded general state-setter:

  Task<bool> SetFunction(StoredId storedId, Status status, byte[]? param, byte[]? result,
      StoredException? exception, long expires, long timestamp,
      ReplicaId? owner,                       // null => release (owner -> NULL)
      IReadOnlyList<StoredEffect>? effects,   // null => leave effects column untouched
      ReplicaId expectedReplica,              // non-nullable: caller must already own the flow
      IStorageSession? storageSession);       // store's forward-carried context (from ClaimFunction)

  Writes status/owner/effects in a single UPDATE guarded by owner = expectedReplica; returns true iff the guard matched. expectedReplica is non-nullable — a caller must own the flow to use it (matching Succeed/Fail). The final storageSession is the store's opaque per-flow context; when supplied it's kept coherent with the persisted effect snapshot, mirroring each store's EffectsStore.SetEffectResults bookkeeping (InMemory advances Version/RowExists/Effects; the SQL stores update Effects/RowExists under owner-based concurrency).
• Surface result bytes on the claim — add Result to StoredFlowWithEffects (StorageSession kept last), populated from result_json (SQL stores) / in-memory state, so a claimer can feed the flow's current result into SetFunction without nulling it.

Validation

• dotnet build — 0 errors, 0 warnings.
• InMemory core suite — 521 passed, 0 failed.
• ClaimFunction + SetFunction — 18 passed each on InMemory / Postgres / SqlServer / MariaDB (incl. a session-threading test asserting the session stays coherent after SetFunction).
• Full PostgreSQL suite — 373 passed, 0 failed.

No behavior change to existing flows — pure rename + additive owner-guarded primitive + additive Result field.

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