This commit refactors the `unsubscribe_tokens` method in the `KiteTickerSubscriber` struct. Instead of directly unsubscribing the tokens and updating the `subscribed_tokens` field, it now uses a match statement to handle the result of the `unsubscribe_cmd` method. If the unsubscribe is successful, the tokens are removed from the `subscribed_tokens` using the `retain` method. If there is an error, it is returned.

The code changes in this commit ensure that the `subscribed_tokens` field is updated correctly when unsubscribing tokens.

Co-authored-by: Kaushik Chakraborty <git@kaushikc.org>

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ref: #3

chore: refactor models

Signed-off-by: Kaushik Chakraborty <git@kaushikc.org>

Ref: #6