fix(agent): don't overwrite existing vscode git auth settings

[zedkipp]

OverrideVSCodeConfigs previously unconditionally set git.useIntegratedAskPass and github.gitAuthentication to false, clobbering any values provided by template authors via module settings (e.g. the vscode-web module's settings block). This change only set these keys when they are not already present, so template-provided values are preserved.

Registry PR #758 fixed the module side (run.sh merges template-author settings into the existing settings.json instead of overwriting the file). But the agent still unconditionally stamped false onto both keys before the script ran, so the merge base always contained the agent's values and template authors couldn't set them to anything else. This change fixes the agent side by only writing defaults when the keys are absent.

#19007

Test Plan

Prerequisites:

Start develop.sh with an external auth provider configured:

CODER_EXTERNAL_AUTH_0_ID=test-github CODER_EXTERNAL_AUTH_0_TYPE=github CODER_EXTERNAL_AUTH_0_CLIENT_ID=fake CODER_EXTERNAL_AUTH_0_CLIENT_SECRET=fake scripts/develop.sh

Use a template with the vscode-web module overriding the git auth settings (ensure version 1.5.0):

    module "vscode-web" {
      count          = data.coder_workspace.me.start_count
      source         = "registry.coder.com/modules/vscode-web/coder"
      version        = "1.5.0"
      agent_id       = coder_agent.main.id
      accept_license = true

      settings = {
        "git.useIntegratedAskPass" = true
        "github.gitAuthentication" = true
      }
    }

Tests:

1. Fresh start: create a workspace, verify the module's true values survive the agent's defaults:
   coder ssh <workspace> -- cat '~/.vscode-server/data/Machine/settings.json'
   Expected: both keys are true.

2. Restart: restart the workspace, verify the agent doesn't clobber them back to false :

   coder restart <workspace>
   coder ssh <workspace> -- cat '~/.vscode-server/data/Machine/settings.json'
   

   Expected: both keys are still true .

[matifali]

With the test added, it looks good to me. Thanks, I thought we solved this issue on the module side in coder/registry#758. But good to see the complete fix. Thanks.