Raise submodule test coverage: 13 files at 0% with generated mocks already in place

Is your feature request related to a problem? Please describe.

Once #3866 / #3940 land, the datasource and metrics-exporter submodules are part of the published coverage figure for the first time — and they dominate it. In the merged profile from run 34580707916 the submodules contribute 6,009 of 8,770 statements (69%) and sit at 74.7%, so submodule coverage is now the main lever on the repo-wide number.

The cheap part of that gap is unusually cheap: 13 files are at exactly 0%, and most of the modules they live in already ship interface.go + generated mock_*.go (gomock). The seams exist; nothing is testing through them. No Docker, no live services, no new abstractions — just table-driven tests against mocks that are already committed.

Describe the solution you'd like

Cover the files below. Each is independent, so this splits cleanly across contributors and makes a good first issue set. Statement counts are from the merged profile, with mock_*.go excluded (the same rule the merge step applies).

file	stmts (all uncovered)	mocks available	notes
pkg/gofr/datasource/file/sftp/file.go	59	✅ mock_interface.go	biggest single win
pkg/gofr/datasource/couchbase/wrappers.go	35	✅ mock_interfaces.go	thin driver wrappers
pkg/gofr/datasource/surrealdb/utils.go	17	n/a	pure funcs: clean, PrettyPrint, isAdministrativeOperation, isCustomNil — zero deps
pkg/gofr/datasource/pubsub/nats/pubsub_wrapper.go	17	✅
pkg/gofr/datasource/influxdb/internal.go	15	✅
pkg/gofr/datasource/couchbase/logger.go	11	✅ mock_logger.go
pkg/gofr/datasource/influxdb/logger.go	10	✅
pkg/gofr/datasource/kv-store/nats/logger.go	8	✅ mock_logger.go	PrettyPrint
pkg/gofr/datasource/pubsub/eventhub/message.go	7	✅ mock_logger.go	Commit
pkg/gofr/datasource/dgraph/metrics.go	7	✅ mock_metrics.go
pkg/gofr/datasource/surrealdb/wrapper.go	5	✅ mock_interface.go	NewDBWrapper, Use, SignIn, Info, GetDB
pkg/gofr/datasource/solr/logger.go	4	✅ mock_logger.go
pkg/gofr/datasource/kv-store/dynamodb/logger.go	1	✅

196 statements, all currently unexecuted.

Tests should follow the repo convention: one test function per method, table-driven over a []struct of cases.

Describe alternatives you've considered

The same coverage could be reached with testcontainers-backed integration tests, but that is a much larger change to CI runtime and flakiness for the same statements — these files are wrappers, loggers and pure helpers whose behaviour is fully observable through the existing mocks.

Additional context

Where the rest of the submodule gap lives, for anyone who wants the larger pieces (these need new tests or a seam, not just mocks — worth separate issues):

module	coverage	uncovered stmts	effect on the repo-wide figure if taken to 90%
datasource/surrealdb	24.8%	228	+2.25pp
datasource/pubsub/eventhub	40.5%	169	+1.60pp
datasource/mongo	34.5%	97	+0.94pp — already fixed by dropping -short in #4201
datasource/couchbase	62.4%	86	+0.72pp
datasource/file/sftp	54.2%	77	+0.69pp
datasource/influxdb	62.0%	78	+0.66pp
datasource/kv-store/nats	45.9%	53	+0.49pp

Two structural notes found while measuring:

1. surrealdb needs one seam before it can move. newDB (surrealdb.go:89) is called directly inside Connect, so Connect, connectToDatabase, setupNamespaceAndDatabase, signIn, authenticateCredentials and HealthCheck are all unreachable from a test. Making newDB a package-level var (or a Client field) that tests swap for the existing MockDB unlocks all of them at once.

2. The pkg/gofr half of the figure has its own denominator problem, unrelated to this issue. The PKG job runs -coverpkg=./pkg/gofr, so the merged profile contains only the 29 top-level pkg/gofr/*.go files (2,290 statements). Everything under pkg/gofr/http, container, logging, service, cmd and the in-repo datasources is absent from the published number entirely. That is the same class of gap #3866 fixed for submodules, one level up, and deserves its own issue.