Added type checks: if (!params[jss::channel_id].isString()) return rpcError(rpcINVALID_PARAMS); and similarly for signature.

[harshitgupta62]

High Level Overview of Change

Fixes #6765

Adds isString() type validation checks before calling asString()
on the channel_id parameter in ChannelAuthorize.cpp and on both
channel_id and signature parameters in ChannelVerify.cpp.

Previously, non-string inputs (e.g. integers, arrays) could be
silently coerced or cause unexpected behavior. Now, invalid types
return rpcINVALID_PARAMS immediately, consistent with the existing
pattern used for the amount field in the same handlers.

[github-actions]

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