Reduce subtransactions in safe_find_or_create_by

What does this MR do?

Reduces subtransactions in safe_find_or_create_by by only opening the transaction when attempting to create a record.

Old code:

1. Open transaction
2. Find record
3. If not found, create record
4. If create not unique, repeat steps 2 and 3

New code:

1. Find record
2. If not found, open transaction and create record
3. If create not unique, repeat step 1

Note that there is a slight behavior change where we do not attempt to create the record twice. I think it's not expected to do that anyway and it's more of a convenience of using retry on the block.

Sample when record does not exist:

[2] pry(main)> UserInteractedProject.transaction { UserInteractedProject.safe_find_or_create_by(user_id: 1, project_id: 1) }
  TRANSACTION (0.3ms)  BEGIN /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:109:in `begin_db_transaction'*/
  UserInteractedProject Load (0.7ms)  SELECT "user_interacted_projects".* FROM "user_interacted_projects" WHERE "user_interacted_projects"."user_id" = $1 AND "user_interacted_projects"."project_id" = $2 LIMIT $3 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["user_id", 1], ["project_id", 1], ["LIMIT", 1]]
  Feature::FlipperGate Load (0.5ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = $1 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["feature_key", "active_record_subtransactions_counter"]]
  TRANSACTION (0.2ms)  SAVEPOINT active_record_1 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/savepoints.rb:11:in `create_savepoint'*/
  UserInteractedProject Create (9.6ms)  INSERT INTO "user_interacted_projects" ("user_id", "project_id") VALUES ($1, $2) /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["user_id", 1], ["project_id", 1]]
  TRANSACTION (0.2ms)  RELEASE SAVEPOINT active_record_1 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/savepoints.rb:19:in `release_savepoint'*/
  TRANSACTION (1.2ms)  COMMIT /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:119:in `commit_db_transaction'*/
=> #<UserInteractedProject:0x00007eff895a2c30 user_id: 1, project_id: 1>

Sample when record exists:

[3] pry(main)> UserInteractedProject.transaction { UserInteractedProject.safe_find_or_create_by(user_id: 1, project_id: 1) }
  TRANSACTION (0.3ms)  BEGIN /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:109:in `begin_db_transaction'*/
  UserInteractedProject Load (0.5ms)  SELECT "user_interacted_projects".* FROM "user_interacted_projects" WHERE "user_interacted_projects"."user_id" = $1 AND "user_interacted_projects"."project_id" = $2 LIMIT $3 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["user_id", 1], ["project_id", 1], ["LIMIT", 1]]
  TRANSACTION (0.3ms)  COMMIT /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:119:in `commit_db_transaction'*/
=> #<UserInteractedProject:0x00007eff8867bd38 user_id: 1, project_id: 1>

We can see that it's a single SELECT without any subtransaction.

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How to setup and validate locally (strongly suggested)

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