Test FastlyPlatformSecretStore read path and write stubs (PR4) (#557)

* Rename crates to trusted-server-core and trusted-server-adapter-fastly

Rename crates/common → crates/trusted-server-core and crates/fastly →
crates/trusted-server-adapter-fastly following the EdgeZero naming
convention. Add EdgeZero workspace dependencies pinned to rev 170b74b.
Update all references across docs, CI workflows, scripts, agent files,
and configuration.

* Add platform abstraction layer with traits and RuntimeServices

Introduces trusted-server-core::platform with PlatformConfigStore,
PlatformSecretStore, PlatformKvStore, PlatformBackend, PlatformHttpClient,
and PlatformGeo traits alongside ClientInfo, PlatformError, and
RuntimeServices. Wires the Fastly adapter implementations and threads
RuntimeServices into route_request. Moves GeoInfo to platform/types as
platform-neutral data and adds geo_from_fastly for field mapping.

- Defer KV store opening: replace early error return with a local
  UnavailableKvStore fallback so routes that do not need synthetic ID
  access succeed when the KV store is missing or temporarily unavailable
- Use ConfigStore::try_open + try_get and SecretStore::try_get throughout
  FastlyPlatformConfigStore and FastlyPlatformSecretStore to honour the
  Result contract instead of panicking on open/lookup failure
- Encapsulate RuntimeServices service fields as pub(crate) with public
  getter methods (config_store, secret_store, backend, http_client, geo)
  and a pub new() constructor; adapter updated to use new()
- Reference #487 in FastlyPlatformHttpClient stub (PR 6 implements it)
- Remove unused KvPage re-export from platform/mod.rs
- Use super::KvHandle shorthand in RuntimeServices::kv_handle()

* Reject host strings containing control characters in BackendConfig

* Validate scheme and host for control characters in BackendConfig

* Address review findings on platform abstraction layer

* Address review findings on platform abstraction layer

* Add config store read path and storage module split

- Split fastly_storage.rs into storage/{config_store,secret_store,api_client,mod}.rs
- Add PlatformConfigStore read path via FastlyPlatformConfigStore::get using ConfigStore::try_open/try_get
- Add PlatformError::NotImplemented variant; stub write methods on FastlyPlatformConfigStore and FastlyPlatformSecretStore
- Add StoreName/StoreId newtypes with From<String>, From<&str>, AsRef<str>
- Add UnavailableKvStore to core platform module
- Add RuntimeServicesBuilder replacing 7-arg constructor
- Migrate get_active_jwks and handle_trusted_server_discovery to use &RuntimeServices
- Update call sites in signing.rs, rotation.rs, main.rs
- Add success-path test for handle_trusted_server_discovery using StubJwksConfigStore
- Fix test_parse_cookies_to_jar_empty typo (was emtpy)
- Make StoreName and StoreId inner fields private; From/AsRef provide all
  needed construction and access
- Add #[deprecated] to GeoInfo::from_request with #[allow(deprecated)] at
  the three legacy call sites to track migration progress
- Enumerate the six platform traits in the platform module doc comment
- Extract backend_config_from_spec helper to remove duplicate BackendConfig
  construction in predict_name and ensure
- Replace .into_iter().collect() with .to_vec() on secret plaintext bytes
- Remove unused bytes dependency from trusted-server-adapter-fastly
- Add comment on SecretStore::open clarifying it already returns Result
  (unlike ConfigStore::open which panics)

Author: prk-Jr