When PostgreSQLFormat is set to CUSTOM but no projection is supplied,
projection() returned (nil, nil). The subsequent field accesses in
dsn() (proj.Host, proj.Port, ...) then panic with a nil pointer
dereference instead of surfacing the misconfiguration.

Return ErrSecretProjectionMissing in that case, and switch the CUSTOM
branch to use the nil-safe getter chain for consistency with the
UNSPECIFIED handling above it.

AI-assisted: Claude Code (claude-opus-4-7)

Errors returned by dsn() (and the projection() helper it calls) had
no package prefix, so they reached the caller as bare strings like
"unknown secret format: -1" or a multi-line errors.Join output. The
sibling error paths in newFromProto wrap with "postgres:", so the
dsn() path was the only one without it.

Wrap the dsn() result once at the call site to keep newFromProto's
error surface uniform. errors.Is on the underlying sentinels keeps
working since fmt.Errorf with %w preserves the chain.

AI-assisted: Claude Code (claude-opus-4-7)

Three small typos in client_test.go:
- "requies" -> "requires"
- "withou" -> "without"
- "clientOptions.dns()" -> "clientOptions.dsn()" in the t.Fatalf
  format string

AI-assisted: Claude Code (claude-opus-4-7)

fix(v2/postgres): reject CUSTOM format without projection

See merge request !503

Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Co-authored-by: Alessio Caiazza <code.git@caiazza.info>