signal not-connected southbound as codes.Unavailable

[alexandernorth]

Problem

When a datastore's southbound interface (device) is not connected, the
transaction path returned a plain error, which reaches gRPC clients as
code = Unknown. Clients can't distinguish this transient condition from a
permanent failure.

Change

Introduce ErrNotConnected and map it to gRPC codes.Unavailable, and
use it where not connected errors were produced previously.

TargetConnectionStatusenum is dropped andTargetStatusnow usessdcpb.TargetStatus` directly:

• TargetStatus.Err() is the single place a connection state becomes
  ErrNotConnected, keeping the details the target collected
• datastoreToRsp now populates Target.StatusDetails
• gnmi compares ConnectivityState() instead of ConnState() strings
• applyIntent prefixes the datastore name

Notes

Enables config-server to retry instead of treating it as permanent.

Behaviour change: the gnmi Set guard checked only t == nil, it now
rejects any non-ready connection, so a Set during CONNECTING or
TRANSIENT_FAILURE fails fast as Unavailable instead of being attempted.

TargetStatus_UNKNOWN is 0, so an unpopulated status now reads as not
connected.

Pairs-with: sdcio/config-server#468

[codecov]

Codecov Report

❌ Patch coverage is 45.94595% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/datastore/target/gnmi/gnmi.go	0.00%	10 Missing ⚠️
pkg/datastore/target/netconf/nc.go	33.33%	3 Missing and 1 partial ⚠️
pkg/server/datastore.go	0.00%	3 Missing ⚠️
pkg/datastore/intent_rpc.go	0.00%	2 Missing ⚠️
pkg/datastore/datastore_rpc.go	0.00%	1 Missing ⚠️

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[henderiw]

lgtm

[henderiw]

LGTM

[steiler]

Follow-up: #481 closes the residual mid-flight transport-drop gap this PR left open in pkg/datastore/target/netconf/nc.go (the three call sites that still detected drops via a fragile strings.Contains(err.Error(), "EOF") and returned the raw, unwrapped error instead of ErrNotConnected).