fix(http): handle JSON encoding errors in responder

[Aggarwalmansi]

Description

The HTTP responder was writing headers before attempting JSON encoding.
When the encoder failed (e.g. math.NaN()), the error was silently ignored,
resulting in a 200 OK response with an empty body and no logs.

This PR ensures JSON encoding is attempted before writing headers and
properly handles encoding failures.

What changed

• Encode response payload into a buffer before writing headers
• Handle JSON encoding errors explicitly
• Return 500 Internal Server Error when encoding fails
• Prevent silent response failures

Why this is needed

Go’s JSON encoder does not support values like NaN or Inf.
Previously, such cases caused the server to respond with a misleading success status.
This change ensures correct HTTP semantics and improves debuggability.

How it was tested

• Manually verified using a handler returning math.NaN()
• Confirmed response returns 500 Internal Server Error instead of 200
• Ran go test ./pkg/gofr/http/...

Related Issue

Fixes #2739