encoding/pem: refactor lineBreaker.Write to handle buffer sizes correctly

[aimuz]

Refactor lineBreaker.Write in encoding/pem to correctly handle cases
where the input buffer size is less than, equal to, or greater than
pemLineLength. This change improves the logic for writing data and
handling line breaks, ensuring correct byte counts are returned.
Additionally, new unit tests have been added to verify behavior with
both small and large buffers.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Zhen Li:

Patch Set 1:

(1 comment)

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