Update contract changes for KIP-848 regex subscription

[Pranav Rathi (pranavrth)]

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Pull request overview

This PR documents contract changes related to KIP-848's regex subscription handling, specifically the shift from client-side to broker-side regex matching with a different regex engine.

• Adds documentation for regex subscription behavior changes in the KIP-848 consumer protocol
• Documents the migration from libc regex engine (client-side) to Google RE2/J engine (broker-side)
• Provides a concrete example of regex pattern incompatibility between protocols

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[Emanuele Sabellico (emasab)]

Thanks for the clarification I proposed some changes.
There's also a change to the example regex ^topic given using ^topic* is also matching topi and we don't want to add this element that creates confusion about the proposed example change.

[Emanuele Sabellico (emasab)]

LGTM, Thanks Pranav!