feat(internal/librarian/java): add entries to versions.txt for new modules

[perashanid]

This PR adds functionality to automatically update versions.txt when new Java modules are generated. This is needed during migration when release-please is still in use.

Changes Made:

Modified generatePomsIfMissing to track newly created modules and call updateVersionsFile after generating POMs
Added updateVersionsFile function that appends entries to versions.txt for new modules
The function handles creating the file if it doesn't exist, appending to existing content, and avoiding duplicate entries
Format used: module-name:0.0.0:version-SNAPSHOT where 0.0.0 is the initial released version for new modules

Technical Implementation:

When new proto-, grpc-, or gapic client modules are generated, their artifact IDs are collected
The updateVersionsFile function reads the existing versions.txt to avoid duplicates
New entries are appended with format: module:0.0.0:library-version-SNAPSHOT
The file is created in the repository root (parent directory of the library directory)

Testing:

Added comprehensive test coverage in TestUpdateVersionsFile with scenarios for:

• Creating new versions.txt file
• Appending to existing file
• Skipping duplicate modules
• Adding multiple new modules at once

All existing pom-related tests pass
No linting or diagnostic issues

Fixes #4927

[gemini-code-assist]

Code Review

This pull request introduces logic to automatically update the versions.txt file when new Java modules are generated. It includes a new updateVersionsFile function and corresponding unit tests. The review feedback suggests simplifying the file-reading logic to avoid redundant system calls and improving test assertions by using cmp.Diff to align with the project's style guide.