feat: changed APIFY_MCP_PROXY_URL to ACTOR_MCP_PROXY_URL (#674)

Based on this discussion in Slack:
https://apify.slack.com/archives/C0AERKHH8AV/p1785147014865919

There will be one more PR to worker that will actually use the constant,
and one to docs.

And one to the whitepaper: https://github.com/apify/actor-whitepaper

Author: valekjo

packages/consts/src/consts.ts

@@ -293,7 +293,6 @@ export const APIFY_ENV_VARS = {
     PROXY_PASSWORD: 'APIFY_PROXY_PASSWORD',
     PROXY_PORT: 'APIFY_PROXY_PORT',
     PROXY_STATUS_URL: 'APIFY_PROXY_STATUS_URL',
-    MCP_PROXY_URL: 'APIFY_MCP_PROXY_URL',
     PURGE_ON_START: 'APIFY_PURGE_ON_START',
     SDK_LATEST_VERSION: 'APIFY_SDK_LATEST_VERSION',
     SYSTEM_INFO_INTERVAL_MILLIS: 'APIFY_SYSTEM_INFO_INTERVAL_MILLIS',
@@ -323,8 +322,12 @@ export const APIFY_ENV_VARS = {
     TIMEOUT_AT: 'APIFY_TIMEOUT_AT',
 
     // Deprecated, keep them for backward compatibility:
+    /** @deprecated */
     ACT_ID: 'APIFY_ACT_ID',
+    /** @deprecated */
     ACT_RUN_ID: 'APIFY_ACT_RUN_ID',
+    /** @deprecated */
+    MCP_PROXY_URL: 'APIFY_MCP_PROXY_URL',
 } as const;
 
 /**
@@ -350,6 +353,7 @@ export const ACTOR_ENV_VARS = {
     INPUT_KEY: 'ACTOR_INPUT_KEY',
     MAX_PAID_DATASET_ITEMS: 'ACTOR_MAX_PAID_DATASET_ITEMS',
     MAX_TOTAL_CHARGE_USD: 'ACTOR_MAX_TOTAL_CHARGE_USD',
+    MCP_PROXY_URL: 'ACTOR_MCP_PROXY_URL',
     MEMORY_MBYTES: 'ACTOR_MEMORY_MBYTES',
     RESTART_ON_ERROR: 'ACTOR_RESTART_ON_ERROR',
     PERMISSION_LEVEL: 'ACTOR_PERMISSION_LEVEL',