InnoDB: Tighten .cfg file parser string length validation

The .cfg metadata file parser in storage/innobase/row/row0import.cc (used by ALTER TABLE ... IMPORT TABLESPACE) uses overly permissive or missing length limits when reading identifier strings from .cfg files.

String lengths are read as 4-byte integers via mach_read_from_4(), meaning a crafted .cfg file can specify lengths up to 2^32 bytes. Without proper bounds, this causes excessive memory allocation via UT_NEW_ARRAY_NOKEY() before the
string content is even validated.

Current state on main:

• Field names: no length validation at all
• Index names: limited to OS_FILE_MAX_PATH (4000), no check for zero
• Column names: limited to 128 with a FIXME: What is the maximum column name length? comment
• Hostname: no length validation at all
• Table name: no length validation at all

Correct limits (all lengths include the NUL terminator):

• Field/index/column names: NAME_LEN + 1 (193), the maximum identifier length
• Hostname: HOSTNAME_LENGTH + 1 (256), consistent with MariaDB's own hostname limit. Note: RFC 1035 defines the textual DNS name limit as 253 characters (253+1=254 with NUL), but MariaDB's HOSTNAME_LENGTH (255) is based on the RFC 1034
  wire-format limit of 255 octets. Using HOSTNAME_LENGTH avoids rejecting .cfg files exported from servers with hostnames between 254-255 characters, which MariaDB itself permits.
• Table name: MAX_FULL_NAME_LEN + 1 (655), since the .cfg file stores the full db/table name (written by row0quiesce.cc as table->name.m_name)

The existing row_import_cfg_read_string() function already validates that the NUL byte appears at exactly position len - 1, which prevents embedded NUL bytes and ensures proper termination.

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https://github.com/MariaDB/server/pull/4913