[Enhancement] Set initial capacity for ProxyContext internal HashMap to avoid resize

### Before Creating the Enhancement Request - [x] I have confirmed that this should be classified as an enhancement rather than a bug/feature. ### Summary `ProxyContext` (in `org.apache.rocketmq.proxy.common`) uses a `HashMap` with the default initial capacity (16) and default load factor (0.75) to hold context entries. The resize threshold is therefore `12`. In typical request paths, the context is populated with the 10 standard keys defined in `ContextVariable` (`remote-address`, `local-address`, `client-id`, `channel`, `language`, `client-version`, `remaining-ms`, `action`, `protocol-type`, `namespace`) plus additional entries injected via `withVal(...)` from various processors / interceptors. As soon as the entry count exceeds 12, the underlying `HashMap` triggers a rehash/resize on the hot request path, which is unnecessary overhead given that the upper bound of entries is well-known. ### Motivation `ProxyContext` is created on every inbound request (gRPC / Remoting), so this resize cost is paid per RPC. Pre-sizing the map eliminates a guaranteed reallocation + rehash on a hot path with effectively zero downside (the extra memory footprint is negligible, ~256 bytes per context). ### Describe the Solution You'd Like Initialize the internal map with an explicit initial capacity (suggest `64`) so that under the default load factor the threshold (`48`) comfortably exceeds the realistic upper bound of context entries, and no resize will happen on the request path. ```java // proxy/src/main/java/org/apache/rocketmq/proxy/common/ProxyContext.java private static final int DEFAULT_INITIAL_CAPACITY = 64; private final Map<String, Object> value = new HashMap<>(DEFAULT_INITIAL_CAPACITY); ``` ### Describe Alternatives You've Considered - Keep using the default capacity — rejected, incurs a deterministic resize on every request once entries cross 12. - Use a smaller capacity such as 32 — works for most cases but still risks resize when interceptors/plugins push more values; 64 leaves comfortable headroom while staying small. - Switch to a different map implementation (e.g. `IdentityHashMap`, array-backed) — out of scope; preserving `HashMap` semantics with a tuned initial capacity is the minimal change. ### Additional Context File: `proxy/src/main/java/org/apache/rocketmq/proxy/common/ProxyContext.java` Related code: - `ProxyContext#value` field — the map being resized. - `ContextVariable` — defines 10 well-known keys, all of which are usually set during a single request lifecycle. Happy to send a PR.