txn_box: Log text block reload failures

[bneradt]

Text blocks silently lose their file-backed content when a periodic
reload can no longer read the configured file. Requests then see an
empty block without an operator-visible diagnostic.

This logs the transition to unavailable content once and adds
regression coverage for a file that becomes unreadable. This also
documents the error and fallback behavior.

The original txn_box repository is defunct, so fixes for the plugin are
now tracked in the Traffic Server repository. This patch addresses:
SolidWallOfCode/txn_box#90

[Copilot]

Pull request overview

This PR improves operator visibility for the experimental txn_box plugin’s file-backed text blocks by logging (once) when periodic reloads lose access to the configured file and the block content is cleared. It also adds a gold test to cover the “file becomes unreadable” scenario and updates the user documentation to describe the error/fallback behavior.

Changes:

• Log a one-time ERROR when a periodic text-block reload fails after previously having file content (transition to unavailable content).
• Add AuTest regression coverage that makes a configured text-block file unreadable during runtime and asserts the error is logged.
• Update directive reference documentation to clarify behavior when the file disappears or becomes unreadable.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
plugins/experimental/txn_box/plugin/src/text_block.cc	Logs the transition from available file content to unavailable content when reload fails, without holding the content lock during logging.
tests/gold_tests/pluginTest/txn_box/static_file/txn_box_static_file.test.py	Adds a runtime permission-change step and asserts the expected error appears in diags.log.
tests/gold_tests/pluginTest/txn_box/static_file/static_file.replay.yaml	Defines an unreadable text block with a short reload period to exercise the failure path.
tests/gold_tests/pluginTest/txn_box/static_file/unreadable.txt	Adds the file used by the new unreadable-file regression scenario.
doc/admin-guide/plugins/txn_box/user/DirectiveReference.en.rst	Documents the logged error and fallback behavior when a file becomes unreadable during update checking.