fix(nats): propagate object store init errors

[corylanou]

Description

Propagates replica client initialization failures from Store.Open() before any database or compaction monitors start. All replica clients complete initialization as a separate pass before any database opens. NATS initialization now considers the object store, rather than only the NATS connection, to be the completed initialization state and reuses an existing connection when retrying object store access.

Motivation and Context

With origin/main at c96c0f4 and a local nats:latest -js server that did not contain the configured object store, this command reproduced issue #1349:

litestream replicate /tmp/test.db nats://127.0.0.1:4222/missing-bucket

Before this change, the first access returned nats: bucket not found after setting ReplicaClient.nc and ReplicaClient.js. A concurrent startup access then saw the non-nil connection, treated initialization as complete, and called objectStore.List() on a nil object store:

panic: runtime error: invalid memory address or nil pointer dereference
nats.(*ReplicaClient).LTXFiles
nats/replica_client.go:285

The focused regression test reproduced the same partial state and panicked before the fix. After the fix, the CLI returns the initialization error once and exits with status 1:

Error: cannot open store: initialize replica client for "/tmp/test.db": nats: failed to initialize object store: failed to access object store bucket "missing-bucket" (bucket must be created beforehand): nats: bucket not found

Fixes #1349

Scope

In scope:

• Initialize replica clients before Store starts database and maintenance monitors.
• Require the complete client initialization pass to succeed before opening any database.
• Return initialization failures to the caller.
• Keep NATS clients retryable after object store initialization fails.
• Cover partial NATS initialization, Store error propagation, and multi-database startup atomicity.

Not in scope:

• Creating NATS object store buckets automatically.
• Changing NATS authentication or TLS configuration.
• Changing background retry behavior after successful startup.

Behavior Change

Scenario	Before	After
NATS bucket is missing	Logs an initialization error, then panics	Returns the initialization error and exits 1
Replica client initialization fails	Starts background work that encounters the error	Store.Open() returns the error before monitors start
NATS connection exists but object store init failed	Treats the client as initialized	Retries object store initialization on the existing connection

How Has This Been Tested?

• go test ./nats -run TestReplicaClient_LTXFiles_InitError -count=1 -v
• go test . -run TestStore_Open_InitError -count=1 -v
• go test -race . -run TestStore_Open_InitErrorDoesNotOpenDBs -count=20
• go test ./... -count=1
• go test -race ./... -count=1
• go vet ./...
• pre-commit run --all-files
• Docker NATS JetStream reproduction with a missing bucket; verified one initialization error, no panic, and exit status 1.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[github-actions]

PR Build Metrics

✅ All clear — no issues detected

Check	Status	Summary
Binary size	✅	36.95 MB (0.0 KB / 0.00%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.12 (latest)
Module graph	✅	1240 edges (0)

Binary Size

	Size	Change
Base (c96c0f4)	36.95 MB
PR (597d4e1)	36.95 MB	0.0 KB (0.00%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 0 vulnerabilities in packages you import and 1
vulnerability in modules you require, but your code doesn't appear to call these
vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	42s
Go version	go1.25.12
Commit	597d4e1

History (1 previous)

Commit	Updated	Status	Summary
c68e90d	2026-07-10 20:00 UTC	✅	36.95 MB (0.0 KB / 0.00%)

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_{🤖 Updated on each push.}