feat: Add GraphQL variables support (#576)

[deepak0x]

PR Description

Adds GraphQL variables support to the request editor.

A dedicated JSON variables editor is shown below the query editor for GraphQL requests. Variables are stored as raw JSON text on HttpRequestModel and merged into the outgoing request body as a parsed JSON object under the standard "variables" key, e.g.:

{
  "query": "query($id: ID!) { user(id: $id) { name } }",
  "variables": { "id": "42" }
}

If the variables text is not valid JSON, the raw string is sent as-is so nothing is silently lost.

This follows the JSON-editor approach recommended by the maintainer on the issue (a simple JSON text editor like the JSON content-type body, rather than a form/key-value editor that parses and rebuilds types), which also keeps the door open for follow-ups like filters & mutations.

Changes

• HttpRequestModel: new variables field + hasVariables getter (codegen regenerated).
• getGraphQLBody: include parsed variables in the body when present.
• Request body UI: labeled Query editor + JSON Variables editor for GraphQL.
• Provider update(): thread variables through.
• Unit tests for variables serialization (valid JSON, empty, invalid JSON).

Related Issues

• Closes GraphQL Variables #576

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: please replace this line with details on why tests have not been included

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

[deepak0x]

@ashitaprasad @animator ready for review. This uses the JSON text editor approach you suggested on #576.