Stricter filename validations

[Mpdreamz]

Summary

Enforces stricter filename validation to ensure consistency across documentation files. Filenames must now:

• Be lowercase
• Start with an alphanumeric character or underscore
• Contain only alphanumeric characters, spaces, dashes, dots, and underscores

This PR adds validation for both the full file path and the filename itself, catching issues like uppercase letters in directory names and invalid starting characters.

Changes

• Added FilePathRegex to validate the full path (lowercase, valid characters)
• Added FileNameRegex to validate the filename starts with [a-z0-9_]
• Combined both validations in IsValidFileName
• Extended test coverage with 140 theory-based test cases

Test plan

• All OutputDirectoryTests pass (140 tests)
• Validation correctly rejects uppercase in paths and filenames
• Validation correctly rejects filenames starting with hyphens, dots, or spaces

Files requiring updates in other repositories

The following files have leading underscores and need to be renamed before we can make the check strictly alphanumeric (removing underscore from allowed starting characters):

elasticsearch

• docs/reference/elasticsearch-plugins/_reimplementing_and_extending_the_analyzers.md
• docs/reference/elasticsearch-plugins/_other_command_line_parameters.md
• docs/reference/elasticsearch-plugins/_plugins_directory.md
• docs/reference/elasticsearch-plugins/_reimplementing_and_extending_the_analyzers_2.md

integrations

• docs/extend/_publish_an_integration.md

docs-content

• deploy-manage/tools/cross-cluster-replication/_failback_when_clustera_comes_back.md
• deploy-manage/tools/cross-cluster-replication/_failback_when_clustera_comes_back_2.md
• deploy-manage/tools/cross-cluster-replication/_prerequisites_14.md
• deploy-manage/tools/cross-cluster-replication/_configure_privileges_for_cross_cluster_replication_2.md
• deploy-manage/tools/cross-cluster-replication/_perform_update_or_delete_by_query.md
• deploy-manage/tools/cross-cluster-replication/_failover_when_clustera_is_down.md
• deploy-manage/tools/cross-cluster-replication/_failover_when_clustera_is_down_2.md
• deploy-manage/tools/cross-cluster-replication/_connect_to_a_remote_cluster.md
• troubleshoot/observability/apm/_collection_of_diagnostic_information.md
• troubleshoot/observability/apm/_agent_is_not_instrumenting_code.md
• reference/fleet/_agent_configuration_encryption.md

beats

• docs/reference/metricbeat/_host_setup.md
• docs/reference/metricbeat/_galera_status_metricset.md
• docs/reference/metricbeat/_live_reloading.md
• docs/reference/metricbeat/_metricsets_70.md
• docs/reference/filebeat/_live_reloading.md
• docs/reference/filebeat/_set_up_the_oauth_app_in_the_salesforce_2.md
• docs/extend/_migrating_dashboards_from_kibana_5_x_to_6_x.md

ecs-dotnet

• docs/reference/_enrichers_2.md
• docs/reference/_usage.md
• docs/reference/_extending_ecsdocument.md
• docs/reference/_formatters.md
• docs/reference/_elasticsearch_security.md

ecs-logging-java

• docs/reference/_structured_logging_with_log4j2.md

go-elasticsearch

• docs/reference/_getting_started_with_the_api.md

elasticsearch-net

• docs/reference/_options_on_elasticsearchclientsettings.md

[theletterf]

Would a doc sweep be a requirement before we merge this?

Thinking of more scenarios such as unique anchors, etc.

[Mpdreamz]

We should do a pass but we don't have to block merging this, we have allow listed the offenders for now.

cc @colleenmcginnis