fix(adapters): defer config validation to call-time (#64)

[techiejd]

Closes #64.

Problem

The mongodb and cf adapter factories threw on missing config at construction time. Because the adapter is instantiated as a top-level statement in payload.config.ts — before Payload (and its logger) exist, and before the plugin's disabled flag is ever consulted — this breaks pnpm generate:types and pnpm generate:importmap in any environment without the runtime variables present (e.g. CI that builds payload-types to publish as a separate package).

Notably, disabled can't work around this: the throw happens in the adapter factory, upstream of the plugin.

Fix

Move validation to call-time (every call, not memoized):

• mongodb (adapters/mongodb/src/index.ts): uri / dbName / pools / dimensions checks now run inside each data method via a getCtx() helper. getConfigExtension (which runs at config-build time) builds the resolved pools defensively and never throws.
• cf (adapters/cf/src/index.ts): removed the factory-time binding is required throw. The existing runtime getVectorizeBinding() check already enforces the binding at call-time for the methods that need it (storeChunk / search / deleteChunks).
• pg: unaffected — its factory takes no env-derived args and never threw at build time.

Tests (TDD)

• New adapters/mongodb/dev/specs/validation.spec.ts (10 tests): construction with missing uri/dbName/pools/dimensions does not throw, getConfigExtension does not throw with fully-missing config, and each data method still throws the right error at call-time.
• Updated the cf validation test from "throws at construction" to "does not throw at construction" + a new "throws at call-time" test (strengthened, not weakened).

All green: mongodb 106/106 (incl. DB-backed compliance), cf adapter 16/16; both adapters typecheck clean.