fix(webapp): keep session runs off the legacy realtime streams backend (#4564)

## Summary

Runs created for a Session were triggered without a realtime streams
version, so they fell through to the `realtimeStreamsVersion` column
default of `v1`. A Session's own `.in` / `.out` channels are always
`v2`, so any run-scoped `streams.append()` or `streams.pipe()` call made
inside a session run wrote to a different backend than the session it
belongs to, and stayed there for the life of the run.

The API trigger routes were never affected. They call
`determineRealtimeStreamsVersion` with the client's
`x-trigger-realtime-streams-version` header and always pass an explicit
value, so a current SDK asking for v2 gets it. Only the internal callers
that build trigger options by hand were leaning on the column default,
which no env var can influence because that path never calls the
resolver at all.

## The version resolver

Fixing the call site exposed a second problem in
`determineRealtimeStreamsVersion`. Its two paths disagreed: an explicit
`v2` was checked against the S2 configuration first, but when the caller
expressed no preference it returned `REALTIME_STREAMS_DEFAULT_VERSION`
verbatim with no check. A deployment that set the default to `v2`
without configuring S2 therefore stamped runs `v2`, nothing failed at
trigger time, and every later read or write against those runs' streams
threw `Realtime streams v2 is required for this run but S2 configuration
is missing` for the life of the run.

Both paths now resolve through one pure function that takes its
configuration rather than reading `env`:

```ts
const requested = streamVersion ?? config.defaultVersion;
if (requested !== "v2") return "v1";

const hasCredentials = Boolean(config.accessToken) || config.skipAccessTokens;
return hasCredentials && Boolean(config.basin) ? "v2" : "v1";
```

## The basin requirement

`resolveStreamBasin` resolves run, session and organization basins ahead
of the global setting, so a deployment that provisions a basin per
organization can serve v2 with no global basin at all. Gating purely on
the global setting would degrade every run there to `v1`.

`determineRealtimeStreamsVersion` therefore takes an optional
organization basin, and every caller that holds one passes it, including
the session path:

```ts
basin: organizationBasinName ?? env.REALTIME_STREAMS_S2_BASIN,
```

This is deliberately the resolved basin and not the
`REALTIME_STREAMS_PER_ORG_BASINS_ENABLED` flag. The flag says the
feature is on, not that a given organization has been provisioned, and
provisioning happens out of band. Keying off the flag would stamp `v2`
on runs for unprovisioned organizations, recreating the failure this
removes.

**This widens behaviour for explicit `v2` requests**, which previously
required the global basin: a provisioned organization on a per-org
deployment now resolves `v2` where it used to get `v1`. That is
intentional, and it makes every path agree.

## Scope

Only newly created runs change. A run already stamped `v1` keeps that
version for its lifetime by design, since readers resolve the backend
from the same column and its existing streams have to stay readable.
Scheduled runs reach the same column default through
`scheduleEngine.server.ts` and are deliberately left alone: that one is
a policy question about `REALTIME_STREAMS_DEFAULT_VERSION` rather than
an inconsistency inside a single feature.

## Verification

A full-stack e2e boots the real webapp plus Postgres, Redis and s2-lite,
creates a Session through the public API so the run comes from the real
trigger path, appends records the way `streams.append()` does, and
asserts three things at once: the version stamped on the run, that the
payload is readable from S2, and that no key exists in Redis. It appends
at a realistic record size so the route's body cap and S2's per-record
cap are both exercised. Reverting the session-path change flips all
three observations, so it fails against the old behaviour rather than
passing vacuously.

Unit tests cover the resolver matrix, including organization-basin-only
and credential-only configurations; two of them fail against the
previous resolver.

Also verified by hand against a local stack: a real `chat.agent` session
run writing 8 records of 250KB through `streams.append()` put 2,049,072
bytes into S2 with no Redis key, while the same agent with the
session-path change removed put 2,102,360 bytes into Redis and nothing
into S2.

.server-changes/streams-version-s2-guard.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: fix
+---
+
+Realtime streams written inside a chat session run now use the same backend as the session itself, and runs are no longer created against a backend that cannot serve them.