postgres_cdc: relax aws.endpoint to accept bare hostnames

[david-yu]

Summary

Follow-up to #4433 (mysql_cdc). The same aws.endpoint papercut exists on postgres_cdc: the AWS SDK's RDS token signer requires host:port but the field is documented (and rendered in --help) as a bare hostname. Users hitting it see:

ERROR  building IAM auth token: the provided endpoint is missing a port,
       or the provided port is invalid

This PR mirrors the mysql_cdc fix:

• internal/impl/postgresql/input_pg_stream.go — endpoint description updated to call out host:port and the auto-derive behaviour. Field is now optional with a "" default.
• internal/impl/postgresql/aws/aws.go — new normalizeIAMEndpoint(endpoint, host, port) helper. If endpoint is empty, use pgconn.Config.Host + Port; if it's a bare hostname, splice in the DSN port (5432 fallback).
• internal/impl/postgresql/aws/aws_test.go — table-driven unit tests for the helper.
• docs/modules/components/pages/inputs/postgres_cdc.adoc — regenerated via task docs.

Only N7 from the operator PR thread applies to postgres_cdc:

• N8 (AllowCleartextPasswords) is MySQL-driver-specific; pgx accepts the IAM token natively.
• N9 (REQUIRE SSL) is MySQL syntax.

UX before / after

Before — bare hostname produces a confusing SDK error:

input:
  postgres_cdc:
    dsn: "postgres://user@mydb.abc123.us-east-1.rds.amazonaws.com:5432/shop?sslmode=require"
    aws:
      enabled: true
      region: us-east-2
      endpoint: mydb.abc123.us-east-1.rds.amazonaws.com   # missing :5432

ERROR  building IAM auth token: the provided endpoint is missing a port,
       or the provided port is invalid

After — the port from the DSN is spliced in (or 5432 as a final fallback). aws.endpoint can also be omitted entirely:

input:
  postgres_cdc:
    dsn: "postgres://user@mydb.abc123.us-east-1.rds.amazonaws.com:5432/shop?sslmode=require"
    aws:
      enabled: true
      region: us-east-2
      # endpoint inherited from the DSN

Backwards compatibility

Fully compatible. host:port values are returned unchanged; the new fallbacks only fire when the user supplied a value that previously errored out.

Existing config	Before	After
aws.endpoint: "host:5432"	works	works (passed through)
aws.endpoint: "host" (bare hostname)	errored at startup	now works
aws.endpoint omitted	rejected at config parse (field was required)	accepted — falls back to DSN
aws.enabled: false (or unset)	unchanged	unchanged

Field schema goes from required to optional with "" default — a relaxation; nothing that previously parsed will now fail.

Test plan

• go test ./internal/impl/postgresql/...
• golangci-lint run ./internal/impl/postgresql/...
• task docs (regenerated postgres_cdc.adoc)
• Integration test against RDS Postgres with aws.enabled: true and a bare-hostname aws.endpoint

🤖 Generated with Claude Code