Validate external file path at metadata load time - #9121
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External tables store the file path in RDB$EXTERNAL_FILE at CREATE TABLE time without any check against the ExternalFileAccess policy. The path was validated only later, when the table was actually opened, so a table pointing outside the allowed directories could be created and the malicious path remained in metadata. Move the validation into a shared checkExternalFileAccess() helper and call it both when the relation metadata is loaded (met.epp) and when the external file is opened (ExternalFile::open). Paths escaping the configured directory list are now rejected at CREATE TABLE time with isc_conf_access_denied, and existing databases with such tables fail on first access instead of silently carrying the bad path.
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External table paths are stored in RDB$EXTERNAL_FILE at CREATE TABLE time without any check against ExternalFileAccess. The path was only validated when the table was opened, so with a restricted directory list a user could create a table pointing outside the allowed directories and the bad path stayed in metadata.
This moves the check into checkExternalFileAccess(), shared by the metadata loader (met.epp) and ExternalFile::open(). Escaping paths are now rejected at CREATE TABLE with isc_conf_access_denied. Existing databases containing such tables fail on first access instead of keeping the invalid path silently.
Tested with ExternalFileAccess = Restrict: CREATE TABLE with a path outside the list fails with SQLSTATE 28000, tables inside the list work unchanged.
Companion tests: FirebirdSQL/firebird-qa#39