Fix Prod issues on /user command

[j24m]

Issue :

• https://github.com/Real-Dev-Squad/discord-slash-commands/issues/147

  Description :

• This pull request introduces improvements to the handling of user details formatting in the formatUserDetails function and addresses the scenario where discordJoinedAt is undefined when we use the /user command.

  Code Changes :

• userCommand.ts : In the userCommand function, it is updated to return an appropriate response when the user details cannot be found.

• formatUserDetails.ts : In the formatUserDetails and convertTimeStamp functions, I've added enhanced handling for discordJoinedAt to ensure the user details are correctly formatted even when discordJoinedAt is undefined.

• tests/fixtures/users.ts : I've introduced a new fixture userWithoutDiscordJoinedAt representing a user without discordJoinedAt data.

• tests/unit/utils/formatUserDetails.test.ts : I've added new test cases to verify the behavior of the formatUserDetails function when discordJoinedAt is undefined.

  Test Stats :

• Before :

• After :

  Working Proof :

• User found :

• User not found : (BOT)

• User not found : (HUMAN)

• Server Join Date (N/A) :

[Pratiyushkumar]

Lgtm

[j24m]

@j24m can we do a small change by change text to Server joined on

Not needed right now in my opinion, already discussed with @ankushdharkar during morning standup. Also it is not change by change to text Server joined on, it's change by changing text to and so on.