[Performance] Avoid N+1 remote table enumeration in external session bypass SHOW TABLES path

## Summary

In the external session-bypass path, `ExternalDatabase.getTables()` performs **N+1 remote table-name enumerations**.

When `extCatalog.shouldBypassTableNameCache(sessionContext)` is true:

1. `getTables()` calls `getTableNamesWithLock()`, which enumerates remote table names once via `listLocalTableNamesWithoutCache(sessionContext)`.
2. Then `getTables()` iterates all table names and calls `getTableNullable(tblName)`.
3. In bypass mode, `getTableNullable(tblName)` enters `getTableNullableWithoutCache(sessionContext, tableName)`.
4. `getTableNullableWithoutCache(...)` calls `findTableNamePairWithoutCache(...)`, which again does a full `listTableNames(sessionContext)` scan for each table.

As a result, listing `N` tables triggers `1 + N` remote `LIST TABLES` enumerations.

## Affected code

`fe/fe-core/src/main/java/org/apache/doris/datasource/ExternalDatabase.java`

Current structure:

```java
public List getTables() {
Set tblNames = getTableNamesWithLock(); // 1 full remote enumeration in bypass mode
for (String tblName : tblNames) {
T tbl = getTableNullable(tblName); // each iteration re-enters bypass path
}
}

public Set getTableNamesWithLock() {
if (extCatalog.shouldBypassTableNameCache(sessionContext)) {
return Sets.newHashSet(listLocalTableNamesWithoutCache(sessionContext));
}
}

private T getTableNullableWithoutCache(SessionContext sessionContext, String tableName) {
Optional> matched = findTableNamePairWithoutCache(sessionContext, tableName);
// findTableNamePairWithoutCache() -> listTableNames(sessionContext)
}
```

## Impact

This affects session-bypass catalogs such as delegated Iceberg REST session catalogs.

For `SHOW TABLES` / `getTables()` on a database with many tables, the current implementation scales poorly because every table object construction repeats a full remote name enumeration.

The issue is performance-only, but the overhead can become significant for large databases or expensive remote catalogs.

## Scope / origin

This is a **pre-existing master/baseline issue**, not introduced by PR #65126.

The bypass structure already existed before the external meta cache refactor. PR #65126 carried it forward and added related session-bypass improvements elsewhere, but did not address this `getTables()` path.

## Suggested fix

In bypass mode, reuse a **single remote name enumeration** inside `getTables()`:

- enumerate `(remoteName, localName)` pairs once,
- build a local lookup map from the result,
- construct table objects directly from that snapshot instead of re-calling `getTableNullable()` for every table.

This keeps bypass semantics unchanged while removing the repeated full scans.

## Related

- PR #65126
- Commit `48291325300 [fix](fe) Avoid unnecessary session table enumeration` addressed a nearby delegated-session existence-check path, but not this `getTables()` N+1 path.

Start in fe/fe-core/src/main/java/org/apache/doris/datasource/ExternalDatabase.java at getTables(), then trace getTableNamesWithLock(), getTableNullableWithoutCache(), and findTableNamePairWithoutCache(). Verify the SHOW TABLES/getTables path for session-bypass catalogs reuses one remote name enumeration while preserving table construction and bypass behavior.

Written by the indexing model from the issue text.