feat(sources/postgres): add configurable pgx query execution mode (#2477)

Adds optional `queryExecMode` to postgres source config, allowing users
to set pgx `DefaultQueryExecMode` for compatibility with external
connection poolers (e.g. transaction pooling).

Supported values:
- cache_statement (default)
- cache_describe
- describe_exec
- exec
- simple_protocol

Implementation details:
- parse DSN with `pgxpool.ParseConfig`
- map `queryExecMode` to `pgx.QueryExecMode*`
- create pool via `pgxpool.NewWithConfig`
- validate config using `oneof` tag
- document new field in postgres source docs
- add parser/validation tests

Tests run:
`go test -v ./internal/sources/postgres -count=1 -vet=off`

Refs #2385

---------

Co-authored-by: Molt (OpenClaw) <noreply@users.noreply.github.com>
Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>
Co-authored-by: Yuan Teoh <yuanteoh@google.com>

docs/en/resources/sources/postgres.md

@@ -133,3 +133,4 @@ instead of hardcoding your secrets into the configuration file.
 | user        |       string       |     true     | Name of the Postgres user to connect as (e.g. "my-pg-user").           |
 | password    |       string       |     true     | Password of the Postgres user (e.g. "my-password").                    |
 | queryParams |  map[string]string |     false    | Raw query to be added to the db connection string.                     |
+| queryExecMode | string | false | pgx query execution mode. Valid values: `cache_statement` (default), `cache_describe`, `describe_exec`, `exec`, `simple_protocol`. Useful with connection poolers that don't support prepared statement caching. |