BUGFIX: Make join operation thread safe

[prasadlohakpure]

Description

Make join operation thread safe
Why:
If task_concurreny is added to join task by a user, ambiugious behaviour may occur due to race condition on shared buffer.

Types of changes

Testing :
file: test/pipelines/join.yaml
With task_concurreny set to 2 or more:

Before changes(is task_concurreny was implemented):
Non deterministic behaviour
e.g. Data written multiple times

2026-02-23 12:28:29 - echo - Abigail|Abner|Abraham ◆ Abram|Ace|Ada ◆ Adalee|Adaline|Adalyn ◆ Adalynn|Adam|Adan ◆ Addilyn|Addison|Adelaide
2026-02-23 12:28:29 - echo - Adele|Adelina|Adeline ◆ Adelyn|Adelynn|Aden ◆ Adhara|Adler|Adley ◆ Adonis|Adrian|Adriana ◆ Aadhya|Aaron|Abby ◆ Abdiel|Abdullah|Abel
2026-02-23 12:28:29 - echo - Adele|Adelina|Adeline ◆ Adelyn|Adelynn|Aden ◆ Adhara|Adler|Adley ◆ Adonis|Adrian|Adriana ◆ Aadhya|Aaron|Abby ◆ Abdiel|Abdullah|Abel ◆ Aadhya|Aaron|Abby|Aaliyah
2026-02-23 12:28:29 - echo - Adele|Adelina|Adeline ◆ Adelyn|Adelynn|Aden ◆ Adhara|Adler|Adley ◆ Adonis|Adrian|Adriana ◆ Aadhya|Aaron|Abby

( Adele appearing multiple times)

After changes:
No loss in data, all records from names.txt present in output have just one occurence

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation and I have updated the documentation accordingly.
• I have added tests to cover my changes.

[Copilot]

Pull request overview

This PR fixes a race condition in the join task that occurred when task_concurrency > 1. Multiple concurrent goroutines were accessing and modifying a shared buffer field on the join struct, causing non-deterministic behavior and data corruption.

Changes:

• Moved buffer storage from struct field to local variable in Run() method to eliminate race condition
• Updated flushBuffer and sendJoinedRecords methods to accept buffer as parameter instead of accessing it from struct

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