Malformed pageToken on list endpoints returns HTTP 500 instead of 400

### Describe the bug

List endpoints (listNamespaces, listTables, listViews) accept an opaque `pageToken` query parameter.
When the value is well-formed Base64 but not a valid Polaris page token (garbage, a truncated token, or a token produced by an incompatible Polaris version), the server answers 500 Internal Server Error and logs an ERROR-level stack trace ("Unhandled exception returning INTERNAL_SERVER_ERROR").
Invalid client input should be a 400.

Root cause: `PageTokenUtil.decodePageRequest` (polaris-core, pagination package) performs Base64 decode + SMILE deserialization without error handling. Failures surface as Jackson 3 `tools.jackson.core.JacksonException` (unchecked) or `IllegalStateException` (unknown token type id).
Neither is handled: `IcebergJsonProcessingExceptionMapper` only maps Jackson 2 `JsonProcessingException`, and `IcebergExceptionMapper` falls to `default -> 500`.
Only the "not Base64 at all" case already yields IllegalArgumentException -> 400.

### To Reproduce

1. Start Polaris (e.g. the RustFS quickstart) and obtain a bearer token.
2. curl -i -H "Authorization: Bearer $TOKEN" \
"http://localhost:8181/api/catalog/v1/quickstart_catalog/namespaces?pageToken=AAAA"
("AAAA" is valid url-safe Base64 but not a serialized page token.)
3. Also reproducible with pageToken=aGVsbG8gd29ybGQ= (plain text) or any truncated real token.

### Actual Behavior

HTTP/1.1 500 with {"error":{"type":"StreamReadException", ...}} (type varies: StreamReadException, DatabindException, MismatchedInputException) and an ERROR log entry per request.

### Expected Behavior

HTTP/1.1 400 Bad Request with message "Invalid page token", no ERROR log

### Additional context

Affected call sites: IcebergCatalogHandler listNamespaces / listTables / listViews.
Policy and generic-table listing will be affected too once #5311 (page-token support there) lands.
Fix and tests are ready; PR to follow.

### System information

Polaris main @ 715cf86ff6 (1.8.0-SNAPSHOT), Iceberg 1.11.0, Jackson 3.2.2.
Backend-independent (reproduced with in-memory and Relational JDBC).

Start with PageTokenUtil.decodePageRequest in the polaris-core pagination package, then inspect IcebergJsonProcessingExceptionMapper and IcebergExceptionMapper. Reproduce with the listed curl requests and trace the IcebergCatalogHandler listNamespaces, listTables, and listViews call sites. Done means malformed but Base64-valid tokens return HTTP 400 with “Invalid page token” and do not emit an ERROR stack trace.

Written by the indexing model from the issue text.