fix(server): report session log persistence and keep followers alive - #61831

Session-log clients can detect disabled persistence, and quiet followers receive keep-alives.
Programmatic clients and orchestrators currently receive a replay watermark even when event payloads are not being retained.

🔍 Origin: observed while following session logs with the default server configuration.

❌ Before

1. A client receives log.synced and cannot tell whether new event payloads will be available.
2. A quiet followed log sends no keep-alives.

✅ After

1. The marker includes persist, the current process's persistence setting. Older servers omit it, which means unknown. Enabling persistence does not recover previously discarded history.
2. Followers receive SSE heartbeat comments every 15 seconds. These never advance the durable cursor, and SSE responses bypass compression.

- synced = { aggregateID, seq }
+ synced = { aggregateID, seq, persist }
- follow = encoded durable events
+ follow = encoded durable events + periodic heartbeat comments

📊 Proof

The full affected test files pass 122 tests, without name filters. One parameterized real-server test covers default/explicit persistence, replay, unknown sessions, gzip, heartbeat timing and subsequent durable events. Disabled-persistence cursor assertions reuse the existing bus test. A separate unpatched run was not performed.

The simplify pass reduced the diff from +246/−24 across 13 files to +163/−21 across 11 files by removing the custom HTTP fixture and duplicate setup. Production code is unchanged by that pass.

This is the first slice; the log still carries durable events only. Follow-up slices:

1. Live status, permission/form requests and usage, with authoritative reconnect snapshots.
2. A managed-server persistence setting and explicit history-gap reporting.

👀 Read: packages/server/src/handlers/session.ts adds comments after the standard SSE encoder, preserving its failure frames; packages/server/src/process.ts excludes SSE before compression. Independent review caught the compression ordering, immediate first tick and missing OpenAPI regeneration; all are addressed.

_{Observed with programmatic clients · base: dev, because these are shared core/server contracts · persistence remains opt-in}

Visual

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Added by the opencode bot (AI-generated).

Related: #42839

Public API: adds optional persist to the log.synced marker and SSE heartbeat comments on followed session logs; SSE responses are no longer compressed (backwards compatible)