[text-auditor] Fix clear user-facing grammar in move_fields errors and Cassandra field text

## Text Improvements

The following user-facing text issues were found in the repository. Each is a low-effort fix.

### 1. `move_fields` runtime error messages are awkward and grammatically incorrect

**File:** `libbeat/processors/move_fields/move_fields.go` (lines 84, 87, 91)

**Current text:**
- `move field read field from parent, sub key: %s, failed: %w`
- `move field delete field from parent sub key: %s, failed: %w`
- `move field write field to sub key: %s, new key: %s, failed: %w`

**Suggested fix:**
- `failed to read field from parent (sub key: %s): %w`
- `failed to delete field from parent (sub key: %s): %w`
- `failed to write field to destination (sub key: %s, new key: %s): %w`

**Why:** These messages are user-visible and currently read as malformed English (`move field read field...`). Rephrasing keeps behavior unchanged while improving clarity.

### 2. Cassandra exported field descriptions use incorrect article (`A integer`)

**File:** `packetbeat/protos/cassandra/_meta/fields.yml` (lines 41, 68)

**Current text:**
- `A integer representing the length of the body of the frame (a frame is limited to 256MB in length).`

**Suggested fix:**
- `An integer representing the length of the body of the frame (a frame is limited to 256MB in length).`

**Why:** `A integer` is an unambiguous grammar error in user-facing exported-field text.

## Suggested Actions

- [ ] Update the three `move_fields` error strings in `libbeat/processors/move_fields/move_fields.go` to the clearer `failed to ...` form.
- [ ] Replace `A integer` with `An integer` in both Cassandra field-description occurrences in `packetbeat/protos/cassandra/_meta/fields.yml` and regenerate/sync derived docs as needed.

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> - [x] expires on Aug 27, 2026, 1:33 PM UTC

Start with the three error strings in libbeat/processors/move_fields/move_fields.go at the referenced lines, then inspect the two descriptions in packetbeat/protos/cassandra/_meta/fields.yml. Update the wording exactly as specified and regenerate or sync derived documentation as needed; done means all five user-facing text occurrences are corrected without changing behavior.

Written by the indexing model from the issue text.