Add file locking tests for SEQUENTIAL, LINE-SEQUENTIAL, and RELATIVE files

[Copilot]

This PR adds comprehensive tests for file locking functionality for SEQUENTIAL, LINE-SEQUENTIAL, and RELATIVE file organizations to tests/run.src/miscellaneous.at.

Changes Made

Test Coverage Added

Added 3 new test cases that verify file locking behavior:

1. File locking - SEQUENTIAL files (Test Run NIST85 tests for Relative files and add the test status to README.md #108)
2. File locking - LINE SEQUENTIAL files (Test Refactor source code based on the result of static analysis #109)
3. File locking - RELATIVE files (Test The output data of DISPLAY statement against data specified PIC PP99 is wrong #110)

Each test verifies all open modes work correctly:

• OUTPUT mode (file creation with shared lock)
• INPUT mode (read access with exclusive lock)
• I-O mode (read/write access with exclusive lock)
• EXTEND mode (append access with exclusive lock)

Implementation Analysis

Through code analysis, confirmed that file locking is already implemented for all three file types in the base CobolFile class:

• Location: libcobj/app/src/main/java/jp/osscons/opensourcecobol/libcobj/file/CobolFile.java
• Method: open_(String filename, int mode, int sharing)
• Technology: Uses Java NIO FileLock with tryLock()
• Error Handling: Returns file status 61 (COB_STATUS_61_FILE_SHARING) when file locking fails

Lock Behavior

// OUTPUT mode uses shared locks, other modes use exclusive locks
boolean lockFlag;
if (sharing != 0 || mode == COB_OPEN_OUTPUT) {
    lockFlag = false;  // Shared lock
} else {
    lockFlag = true;   // Exclusive lock
}

Documentation

Added doc/file-locking-implementation.md documenting:

• Implementation details and lock type determination
• Error handling and file status codes
• Test coverage and scenarios

The tests demonstrate that file locking functionality exists and works correctly for SEQUENTIAL, LINE-SEQUENTIAL, and RELATIVE files as requested.

Fixes #709.

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