[Bug] PostgreSQL data source receives MySQL-style SQL (backticks, CONVERT_TZ, DATE_FORMAT) in multi-datasource setup

### Describe the bug

In a **multi-datasource** configuration, when a cube uses `data_source: 'postgres'` (or similar) and the `driverFactory` correctly returns a **PostgresDriver** instance, the **generated SQL still uses MySQL dialect** instead of PostgreSQL. As a result, queries fail with syntax errors when executed against PostgreSQL.

**Observed MySQL syntax in generated SQL (sent to Postgres):**

- **Identifier quoting**: Backticks (`` ` ``) instead of double quotes (`"`)
- **Functions**: `CONVERT_TZ()`, `convert_tz()`, `DATE_FORMAT()`
- **Session**: `@@session.time_zone`
- **Types/casts**: `TIMESTAMP()`, `CAST(... AS DATETIME)`, `UNIX_TIMESTAMP()`

**Expected:** The schema-compiler / query generator should use the **target data source's dialect** (here: PostgreSQL) when building SQL, so that:

- Identifiers are quoted with `"` instead of `` ` ``
- `CONVERT_TZ` → `(... AT TIME ZONE 'tz') AT TIME ZONE 'tz'`
- `DATE_FORMAT` → `TO_CHAR(..., 'format')`
- `@@session.time_zone` → `'UTC'` or equivalent
- `TIMESTAMP()` / `CAST(... AS DATETIME)` / `UNIX_TIMESTAMP()` → PostgreSQL equivalents

### To Reproduce

**Environment:**

- `@cubejs-backend/server-core`: 1.6.1 (also checked 1.6.4; issue persists)
- `@cubejs-backend/postgres-driver`: 1.6.1
- `@cubejs-backend/mysql-driver`: 1.6.1

**Configuration (`cube.js`):**

- `driverFactory` returns:
- **MySQLDriver** for `dataSource === 'default'` (or when not `postgres`)
- **PostgresDriver** for `dataSource === 'postgres'` (or `postgresDataSource`)
- Cubes with `data_source: 'postgres'` exist and use `sql_table`, measures, dimensions, and time dimensions.

**Steps:**

1. Define a cube with `data_source: 'postgres'` and a `time` dimension (e.g. `granularities: ['day', 'month']`).
2. Send a `/cubejs-api/v1/load` (or equivalent) query that hits that Postgres cube.
3. Inspect the SQL passed to `PostgresDriver.query()` (e.g. via logging or a patch).

**Result:** The SQL contains MySQL syntax (backticks, `CONVERT_TZ`, `DATE_FORMAT`, `@@session.time_zone`, `TIMESTAMP()`, `CAST(... AS DATETIME)`, `UNIX_TIMESTAMP()`), causing PostgreSQL to reject it.

### Workaround

We apply a **monkey-patch** on `PostgresDriver.prototype.query` and `executeQuery` to transform the SQL before execution:

1. Replace backtick `` ` `` with double quote `"`
2. Replace `CONVERT_TZ` / `convert_tz` / `@@session.time_zone` with PostgreSQL `AT TIME ZONE` expressions
3. Replace `DATE_FORMAT` with `TO_CHAR`
4. Replace `TIMESTAMP()`, `CAST(... AS DATETIME)`, `UNIX_TIMESTAMP()` with PostgreSQL equivalents

This works but is fragile and must be reapplied when upgrading; we'd prefer an upstream fix.

### Related

- **#6890** ("Can't find dialect for 'datasource1' data source: undefined"): different symptom (dialect not resolved at all). Our case: dialect/driver is Postgres, but **generated SQL uses MySQL dialect**.
- Docs say Cube uses `dbType` / connection to choose identifier quoting and dialect; in our multi-datasource + Postgres setup, the compiler appears to use a default (MySQL) dialect for SQL generation.

### Root cause hypothesis

The **schema-compiler** (or the layer that generates SQL for a query) may:

- Use a **single, default dialect** (e.g. MySQL) when compiling, regardless of the cube's `data_source`, or
- Resolve `data_source` for the **driver** but not pass the correct **dialect** into the SQL generator, so identifier quoting and function translation still follow the default.

We have not confirmed the exact call path; the above is inferred from "PostgresDriver receives MySQL-style SQL."

### Additional context

- We use both **driverFactory** and `data_source` on cubes; the Postgres driver and connection work; the failure is purely from **SQL syntax**.
- The same cube definition works when the **default** (and only) data source is Postgres; the problem appears in **multi-datasource** when the default is MySQL and a non-default data source is Postgres.

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**References:**
- [Multiple data sources](https://cube.dev/docs/product/configuration/multiple-data-sources)
- [Postgres data source](https://cube.dev/docs/product/configuration/data-sources/postgres)