[Cosmos] Prime metadata caches

[simorenoh]

Prime account properties, container properties, and partition key range caches
eagerly so the first data-plane operation doesn't incur unexpected latency from
cold cache population.

Fixes #3793

Changes

Cache priming in build() (cosmos_client_builder.rs):

• Call refresh_location(false) during build() to eagerly fetch account
  properties and populate regional endpoints before the first operation.
• This also triggers the PPAF callback, configuring partition-level failover
  flags immediately.

Cache priming in container_client() (container_client.rs, database_client.rs):

• ContainerClient::new() eagerly fetches container properties and partition
  key ranges at construction time.
• Exposes partition_key_definition() and container_rid() accessors on
  ContainerClient since the data is already available after priming.
• DatabaseClient::container_client() now returns azure_core::Result<ContainerClient>
  since construction makes fallible HTTP calls.

Signature change propagation (examples, tests, native FFI):

• All container_client() call sites updated with ? to handle the new
  Result return type across examples, emulator tests, and the native
  FFI wrapper.

Other Changes

• Builder tests now use the fault injection framework to mock the HTTP call
  instead of a #[cfg(test)] transport override.