test: exercise FDv2 and FDv1 fallback via the v3 contract tests

[kinyoklion]

Stacked on #314 (the Flutter FDv2 exposure). Wires the Flutter contract test service for the v3 (FDv2) harness and runs it in CI, so the data system — including FDv1 fallback — is exercised end-to-end.

Changes

• Contract service: advertise the fdv1-fallback capability; map the harness dataSystem configuration onto LDConfig.dataSystem (including the fdv1Fallback polling config); regenerate the service API model (service_api.openapi.*) for the dataSystem schema.
• CI (.github/actions/ci/action.yml): add an FDv2 contract-tests run (the launchdarkly/gh-actions/actions/contract-tests action with version: v3, branch: v3) alongside the existing FDv1 run. Because the FDv1 run stops the test service at the end, the FDv2 step restarts it. Modeled on cpp-sdks' contract-tests-fdv2 job.

Validation

The v3 harness passes end-to-end against this service: 816 total / 792 ran / 24 skipped, exit 0 (run locally against this branch on merged main), including the full FDv1 fallback suite. Contract service analyzes clean.

Notes for review

The CI YAML can't be run locally, so a couple of assumptions to sanity-check:

• The pinned contract-tests@contract-tests-v1.3.0 action supports version: v3 / branch: v3 (cpp uses the same action with v3).
• FDv1 and FDv2 share testharness-suppressions.txt (the v3 run passes with it; cpp keeps a separate *-fdv2.txt — we can split if preferred).

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Note

Low Risk
Changes are limited to CI and the contract-test adapter; production SDK behavior is exercised indirectly via harness mapping, with no auth or customer-facing API changes in this diff.

Overview
Adds FDv2 contract test coverage in shared CI alongside the existing FDv1 run: the FDv1 step is labeled explicitly, then a second pass restarts the Flutter contract test service and invokes contract-tests with version/branch v3 (same suppressions file).

The Flutter contract test service is extended for the v3 harness: it advertises fdv1-fallback (and related) capabilities, accepts a dataSystem block on client creation (OpenAPI + codegen), and _mapDataSystem translates harness shapes (connection mode overrides, initializers/synchronizers, fdv1Fallback) into LDConfig.dataSystem and initial connection mode. Smaller harness fixes include omitting null evaluation value keys, stricter context casts, and Timeout.none on the long-running test server.

pubspec.lock bumps pinned local SDK packages used by the contract app.

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