feat: add OAuth login profiles

[Mukil Loganathan (langchain-infra)]

Follow-up to #87, which was merged into mukil/profile-management. This PR retargets the OAuth login layer directly onto main.

Summary:

• add langsmith login with OAuth device-code flow
• store OAuth access/refresh tokens under the selected profile
• refresh profile OAuth tokens when they are missing or near expiry
• use OAuth access tokens as Authorization bearer headers for CLI and raw API commands

OAuth profile format:

{
  "current_profile": "local",
  "profiles": {
    "local": {
      "api_url": "http://localhost:1980",
      "workspace_id": "00000000-0000-0000-0000-000000000000",
      "oauth": {
        "access_token": "REDACTED_OAUTH_ACCESS_TOKEN",
        "refresh_token": "REDACTED_OAUTH_REFRESH_TOKEN",
        "expires_at": "2026-04-29T18:00:00Z"
      }
    }
  }
}

Example commands:

langsmith login --api-url http://127.0.0.1:1980 --profile local --workspace-id 593d649c-2548-4069-af4b-c8867e279ee6 --no-browser

{
  "oauth_expires_at": "2026-04-30T01:08:51Z",
  "profile": "local",
  "status": "logged_in",
  "workspace_id": "593d649c-2548-4069-af4b-c8867e279ee6"
}

langsmith --profile dev --format pretty profile list

  ACTIVE    NAME     API URL                              WORKSPACE ID                            AUTH     EXPIRES AT
---------+-------+-------------------------------------+--------------------------------------+-------+----------------------
  *         dev      https://dev.api.smith.langchain.com    d6684905-655c-429b-bd14-ba302d94c03b    oauth    2026-04-30T02:50:40Z
            local    http://127.0.0.1:1980                  593d649c-2548-4069-af4b-c8867e279ee6    oauth    2026-04-30T01:08:51Z

langsmith --profile dev api GET "/api/v1/sessions?limit=1" --include

HTTP/2.0 200 OK
X-Pagination-Total: 2
Content-Type: application/json

[
  {
    "name": "integ-test-zjgj-96959",
    "id": "8d565e27-6157-44ad-a694-9ab6fdc87e82",
    "tenant_id": "d6684905-655c-429b-bd14-ba302d94c03b"
  }
]

Verification:

• go test ./internal/cmd -run 'TestProfileDisplayTrimsEnvProfile|TestWorkspace' -count=1 on main
• go test ./... -count=1 on mukil/oauth-login-split