Separate GreenPlum connection logic from PostgreSQL package

[Copilot]

PostgreSQL users see pg_session_role errors when postgres.Connect() attempts GreenPlum-specific parameter fallbacks (gp_role, gp_session_role) that don't exist in vanilla PostgreSQL. GreenPlum segments fail to connect in utility mode during backup initialization.

Changes

Extracted GreenPlum connection logic

• Created greenplum/connect.go with GP-specific Connect() that tries standard connection → gp_role=utility → gp_session_role=utility
• Removed tryConnectToGpSegment() from postgres/connect.go
• Updated 6 greenplum package files to use local Connect() instead of postgres.Connect()

Implemented connection function injection

• Added PgConnectFunc type alias for connection function signature
• Made BackupArguments.connectFunc field injectable
• Created NewBackupHandlerWithConnect() to accept custom connection during initialization
• Modified GetPgServerInfo() to accept optional connection function (variadic for backward compatibility)

Fixed segment backup initialization

• NewSegBackupHandler() now injects greenplum.Connect via NewBackupHandlerWithConnect()
• Ensures GP utility mode parameters used throughout handler lifecycle, including during GetPgServerInfo() calls

// Before: hard-coded postgres.Connect() in initialization
func NewBackupHandler(arguments BackupArguments) (*BackupHandler, error) {
    pgInfo, _, err := GetPgServerInfo(false)  // Uses postgres.Connect()
    ...
}

// After: injectable connection with clean delegation
func NewBackupHandlerWithConnect(arguments BackupArguments, connectFunc PgConnectFunc) (*BackupHandler, error) {
    if connectFunc != nil {
        arguments.connectFunc = connectFunc
    }
    pgInfo, _, err := GetPgServerInfo(false, arguments.connectFunc)
    ...
}

// Segment handler now passes GP connection from start
bh, err := postgres.NewBackupHandlerWithConnect(arguments, greenplum.Connect)

PostgreSQL connections no longer attempt GreenPlum-specific fallbacks. GreenPlum functionality preserved with proper utility mode connection handling.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor PG connection from GP stuff</issue_title>
<issue_description>### What feature would you like?

Users (x4m) are very confused to see pg_session_role errors when connecting to Postgres.

Why is this feature useful?

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Refactor PG connection from GP stuff #26

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