bazel: use libxml2 from the BCR

[travisdowns]

Cherry-pick of the libxml2 commit from #28975, updated to use 2.15.2
(matching the version on dev).

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

• none

[Copilot]

Pull request overview

This PR migrates the Bazel libxml2 dependency from a custom http_archive + wrapper BUILD/headers to a Bazel Central Registry (BCR) module dependency, simplifying third-party maintenance.

Changes:

• Adds bazel_dep(name = "libxml2", version = "2.15.1") and removes the prior use_repo-based libxml2 wiring.
• Replaces wrapper includes with direct libxml2 headers (<libxml/parser.h>, <libxml/xmlwriter.h>) and updates Bazel deps to @libxml2.
• Removes the custom third-party BUILD file and wrapper headers for libxml2.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
MODULE.bazel	Adds BCR libxml2 module dep; removes use_repo for the previous non-module libxml2 repo.
MODULE.bazel.lock	Locks BCR module resolution for libxml2 (and associated dependency graph changes).
bazel/repositories.bzl	Removes the manual http_archive fetch of libxml2.
bazel/thirdparty/libxml2.BUILD	Deletes the custom foreign_cc build definition used by the prior http_archive.
src/v/cloud_storage_clients/xml_sax_parser.h	Switches to direct <libxml/parser.h> include.
src/v/cloud_storage_clients/BUILD	Replaces //src/v/thirdparty/libxml2 with @libxml2 in deps.
src/v/test_utils/boost_result_redirect.cc	Switches to direct <libxml/xmlwriter.h> include.
src/v/test_utils/BUILD	Replaces //src/v/thirdparty/libxml2 with @libxml2 in implementation deps.
src/v/thirdparty/libxml2/BUILD	Removes the local wrapper cc_library target.
src/v/thirdparty/libxml2/parser.h	Removes wrapper header.
src/v/thirdparty/libxml2/xmlwriter.h	Removes wrapper header.

[travisdowns]

Rebased on dev and regenerated MODULE.bazel.lock.

[travisdowns]

Updated to use libxml2 2.15.2 (matching what dev already had) via archive_override from the upstream GNOME tarball, since BCR only publishes up to 2.15.1. A patch overlay adds the Bazel build files (BUILD.bazel, MODULE.bazel, config.h, xmlversion.h) derived from the BCR's 2.15.1.bcr.1 module. 2.15.2 was needed for several CVE fixes including CVE-2026-0990.

[travisdowns]

Also pushed .2 upstream to BCR, we can switch to that if/when pulled.

[travisdowns]

Simplified: libxml2 2.15.2 is now published on BCR (bazelbuild/bazel-central-registry#8453 merged), so we can use it directly via bazel_dep. Dropped the archive_override and overlay patch.

[vbotbuildovich]

CI test results

test results on build#83394

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FAIL	src/v/cloud_topics/level_zero/tests/l0_object_size_distribution_test	src/v/cloud_topics/level_zero/tests/l0_object_size_distribution_test		unit	https://buildkite.com/redpanda/redpanda/builds/83394#019dab53-21a0-4b14-b880-472f31dd28f2	0/1