## Which issue does this PR close?

- Closes apache#18452

## Rationale for this change

Previously, the `SHOW` command could not display runtime configuration
settings such as `datafusion.runtime.memory_limit`,
`datafusion.runtime.max_temp_directory_size`, etc. When users attempted
to view these settings, they would receive an error:

Error during planning: 'datafusion.runtime.memory_limit' is not a
variable which can be viewed with 'SHOW'

This PR enables users to query runtime settings using the `SHOW`
command, making them accessible in the same way as other DataFusion
configuration variables.

## What changes are included in this PR?

1. **Added `config_entries()` method to `RuntimeEnv`** - Returns runtime
configuration as `ConfigEntry` objects with human-readable values (e.g.,
"100M", "unlimited")

2. **Exposed getter methods in `DiskManager`** - Added
`max_temp_directory_size()` and `temp_dir_paths()` to access disk
manager configuration

3. **Updated `InformationSchema`** - Modified `make_df_settings()` to
include runtime configuration entries from `RuntimeEnv`

4. **Modified SQL statement validation** - Added check for
`datafusion.runtime.*` variables to allow SHOW command


## Are these changes tested?

Yes, comprehensive tests have been added:

- **set_variable.slt** - Tests SHOW and SET for all 4 runtime variables
(memory_limit, max_temp_directory_size, metadata_cache_limit,
temp_directory)
- **information_schema.slt** - Verifies runtime variables appear in
`information_schema.df_settings`
- Tests cover default values, setting custom values, and querying via
information_schema

## Are there any user-facing changes?

Yes, users can now:

**View runtime settings with SHOW**:
   ```sql
   SHOW datafusion.runtime.memory_limit;
   -- Returns: datafusion.runtime.memory_limit | unlimited