[EM] Remove text file input for external memory.

[trivialfis]

Related: #9472 .

[Copilot]

Pull Request Overview

This PR removes support for text-file-based external memory, standardizes in-memory and external-memory test utilities, and updates documentation and examples accordingly.

• Deprecate and remove “cache” suffix parsing in DMatrix::Load; always load data in-memory or via new iterator-based external memory API
• Introduce GetExternalMemoryDMatrixFromData helper and refactor tests to use RandomDataGenerator and HostDeviceVector for external-memory scenarios
• Remove legacy ExternalMemory examples from JVM packages and update tutorials to reflect removal of text-file external memory

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/data/data.cc	Disallow “#cache” URI syntax and simplify DMatrix::Load
tests/cpp/helpers.{h,cc}	Add GetExternalMemoryDMatrixFromData for tests
tests/cpp/data/test_sparse_page_*	Refactor to use iterator API and RandomDataGenerator
tests/cpp/common/test_hist_util.*	Migrate to HostDeviceVector and remove deprecated helpers
jvm-packages/.../ExternalMemory.*	Remove ExternalMemory example code and references
doc/tutorials/*.rst	Update guidance to note removal of text-file external memory

Comments suppressed due to low confidence (4)

doc/tutorials/external_memory.rst:521

• [nitpick] This standalone bullet may render incorrectly in RST. Consider using a proper directive (e.g., .. note:: or a paragraph) and ensuring a blank line before the item for correct formatting.

- The text file cache format has been removed in 3.1.0.

tests/cpp/data/test_sparse_page_dmatrix.cu:11

• The test uses FileExists but no longer includes the header that defines it (previously provided by filesystem.h). Please reintroduce the appropriate include (e.g. "../filesystem.h") or add a forward declaration for FileExists.

#include "../helpers.h"

tests/cpp/data/test_sparse_page_dmatrix.cc:16

• This file uses dmlc::TemporaryDirectory but the include for filesystem.h (which defines TemporaryDirectory) was removed. Add back the include for filesystem.h or <dmlc/filesystem.h>.

#include "../helpers.h"

src/data/data.cc:1007

• [nitpick] The parameter name for the cache prefix was removed, making the signature less self-documenting. Consider restoring the parameter name (e.g., cache_prefix) for clarity.

template DMatrix* DMatrix::Create(

[trivialfis]

cc @rongou .