fix(synthetics_global_variables): key on (name, type) to resolve 409 conflict

[michael-richey]

Summary

Resolves intermittent 409 Conflict on `datadog-sync sync` for `synthetics_global_variables`:

409 — "Synthetics variable with same name and type already exists"

Root cause. `datadog_sync/model/synthetics_global_variables.py:36` keyed `_existing_resources_map` by `name` only, but the Datadog API enforces uniqueness on the `(name, type)` tuple. When the destination had two variables sharing a name across different types (e.g. `type=variable` and `type=secret_token`), only one survived the last-write-wins collision in the prefetch map. The source variable matching the other type fell through to `POST` and hit the 409.

Fix. Switch `resource_mapping_key` to a composite lambda `lambda r: f"{r['name']}:{r['type']}"`. The existing prefetch + remap path in `create_resource` then resolves conflicts before any POST is issued — no new try/except, no new API call. This mirrors the pattern `Teams` already uses at `datadog_sync/model/teams.py:34` for its `(name, handle)` uniqueness, with the key difference that Synthetics is v1-flat (lambda reads `r['name']` / `r['type']` directly, not `attributes.name` / `attributes.handle`).

Tests

Six unit tests in `tests/unit/test_synthetics_global_variables.py`:

1. `map_existing_resources` retains both same-name variants under distinct composite keys (red→green).
2. Routing assertion — source X/secret_token remaps to the correct destination ID and PUTs, post never called (red→green).
   3a. Composite key resolves to `"X:variable"` on full input (red→green).
   3b. Missing `type` returns `None` silently — guards `base_resource.py:111-115` silent-fail path against future refactors.
3. Unique-name regression — green/green, no behavior change for non-colliding names.
4. Partial-collision (the literal production bug shape) — source X/secret_token with destination only having X/variable correctly POSTs.

All 6 pass; full `tests/unit/` suite (576 tests) green; `tox -e ruff,black` clean.

Cassette note

Existing integration test `tests/integration/resources/test_synthetics_global_variable.py` is unaffected: the destination state fixture at `resources/destination/synthetics_global_variables.json` contains a single variable with a unique name, so the new composite key collapses to the same single-entry map as the old name-only key. No cassette re-record required. (The current cassettes have pre-existing playback failures on `main` unrelated to this fix.)

Out of scope

• No 409 try/except fallback in `create_resource`. Users/roles don't have one; the prefetch path is sufficient. A future PR can layer a 409-race handler if observed in production.
• Type change between syncs. If a source variable's `type` is manually flipped between syncs, the composite key won't match the cached entry and the code will POST a new variable. Acceptable per current semantics — `type` is part of identity at the API level.

Test plan

• `tox -e py313 -- tests/unit/test_synthetics_global_variables.py` — 6/6 pass
• `tox -e py313 -- tests/unit/` — 576/576 pass
• `tox -e ruff,black` — clean

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