Change Description

Add context.Context support to rpcclient's HTTP POST mode, allowing callers to cancel or timeout individual RPC calls. Fixes #2499.

Currently, rpcclient methods like GetBlockCount() don't accept a context, which means callers have no way to cancel or set deadlines on individual RPC calls. In HTTP POST mode, handleSendPostMessage uses http.NewRequest without context, and the 10-retry loop doesn't respect external cancellation.

Changes

• Add ctx field to jsonRequest struct
• Use http.NewRequestWithContext in handleSendPostMessage when ctx is set
• Respect context cancellation during retry backoff (select on reqCtx.Done())
• Add SendCmdWithContext(ctx, cmd) to Client
• Add GetBlockCountWithContext(ctx) as the first context-aware method variant
• Existing methods are unchanged — fully backwards compatible

Design decisions

• SendCmdWithContext mirrors SendCmd with an added ctx parameter
• When ctx is nil (e.g., from existing SendCmd callers), context.Background() is used as fallback
• Context cancellation during retry backoff returns immediately instead of waiting for next attempt
• Additional *WithContext methods for other RPC calls can be added incrementally in follow-up PRs

Steps to Test

go test ./rpcclient/ -run TestGetBlockCountWithContext -v
go test ./rpcclient/ -run TestSendCmdWithContext -v
go test ./rpcclient/ -run TestGetBlockCount_Backwards -v

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.
• 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.