Fix GPT-6-Astra Fast tier description for 0.153.2 (#42632)

## Why

Correct the bundled GPT-6-Astra Fast tier wording for the 0.153.2 patch
release.

## What

Change the Fast tier description from `1.5x speed, increased usage` to
`2x speed, increased usage`. No other catalog fields change.

## ELI5

This updates the text users see. It does not change how requests run or
guarantee an exact speedup.

Manual validation: compared the complete parsed catalog against
`release/0.153` and confirmed only this description changes; `git diff
--check` passed. All 48 `codex-models-manager` library tests passed.

codex-rs/models-manager/models.json

@@ -160,7 +160,7 @@
         {
           "id": "priority",
           "name": "Fast",
-          "description": "1.5x speed, increased usage"
+          "description": "2x speed, increased usage"
         }
       ],
       "additional_speed_tiers": [