rpc: enforce BIP 145 SegWit rules in getblocktemplate

[TechLateef]

Change Description

This PR addresses issue #1738 by enforcing BIP 145 SegWit rules within the getblocktemplate RPC response.

Specifically, this PR updates rpcserver.go to ensure that:

• Client requests are rejected with ErrRPCInvalidParameter if SegWit is active but the client does not explicitly include the segwit rule in their request.
• The rules array in the response conditionally includes !segwit if the generated block template contains transactions with witness data (indicating a witness commitment is required).
• The rules array includes segwit (without the ! prefix) when SegWit is active but the template does not contain any witness transactions.

Steps to Test

1. Run the RPC server integration tests using go test -v -tags=rpctest -run=TestRpcServer ./integration
2. Ensure the newly added testGetBlockTemplateSegwitActiveNoRule, testGetBlockTemplateSegwitActiveWithRule, and testGetBlockTemplateResponseRules tests pass successfully.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.