chore(spec): re-sync vendored spec to the corrected v1 JSON map value schema (PR #112)

[BSalaeddin]

Summary

• Re-syncs tests/fixtures/openapi.json to sendly monorepo PR #112, which widens EventTrackV1.data and WorkflowExecutionStartV1.context map values from type: "object" to the full JSON-type union (string | number | boolean | object | array | null). No endpoints changed.
• Spec-copy only: this SDK types request bodies as loose Mapping[str, Any] / dict[str, Any] by design (see src/sendly/types.py module docstring), so there's no equivalent workaround to remove — zero runtime change.

Test plan

• pytest — 167 passed (includes test_contract.py against the new fixture)
• ruff check . clean
• ruff format --check . clean
• mypy src clean
• python -m build succeeded (sdist + wheel)

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