Fix: Handle JSON array format for chaincode arguments in REST API

[SurbhiAgarwal1]

Summary

This PR fixes a chaincode invocation failure where requests from UI clients resulted in an endorsement error:
rpc error: code = Aborted desc = failed to endorse transaction.

This was caused by a mismatch in how arguments (args) were encoded between the UI (sending a single JSON array) and the Go REST API (expecting repeated form-data fields).

##Root Cause

The Go backend was using r.Form["args"] to read arguments.

• Working (curl): Sends repeated fields args=val1&args=val2, which Go correctly parses into a slice ["val1", "val2"].
• Failing (UI): Often sends a single field args=["val1", "val2"]. Go receives this as a single string ["[\"val1\", \"val2\"]"], causing the chaincode to receive incorrect input and fail endorsement.

Fix & Improvements

1. Backend Robustness (Primary Fix):
   Updated invoke.go to handle both formats. If args is received as a single string, the API now attempts to parse it as a JSON array. If parsing succeeds, it uses the extracted values; otherwise, it falls back to the original behavior.
2. Validation:
   Added a check to return a clear error message if args is empty, preventing silent failures.
3. UI Alignment:
   Aligned the request logic to support modern UI integration patterns (JSON array strings) without breaking existing curl or CLI scripts.

Why this matters

• Improved DX: Developers building UIs on top of these samples won't face silent request formatting issues.
• Backward Compatibility: Existing automation scripts and curl commands continue to work exactly as before.
• Robustness: Better error reporting for missing arguments.

Testing Performed

Verified the following scenarios locally:

• Scenario 1: Repeated form fields (args=val1&args=val2) -> Success
• Scenario 2: JSON array string (args=["val1","val2"]) -> Success
• Scenario 3: Invalid JSON/Single string fallback (args=val1) -> Success
• Scenario 4: Empty arguments -> Correct Error Returned
• Regression: Verified that /query functions (like GetAllAssets) are unaffected.

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Note to Reviewer: This change is minimal and focused on making the REST API more flexible for various client implementations while maintaining the core logic.