[text-auditor] Fix misleading alter-field processor runtime error text

## Text Improvements

The following user-facing runtime error text issues were found in `libbeat/processors/actions/alterFieldProcessor.go`. Both are low-effort fixes in adjacent code.

### 1. Contradictory failure wording in alter error

**File:** `libbeat/processors/actions/alterFieldProcessor.go` (line 162)
**Current text:** `"could not alter %s successfully, %w"`
**Suggested fix:** `"could not alter %s: %w"`
**Why:** This is a failure path, so `could not ... successfully` is self-contradictory and confusing in user-facing output.

### 2. Error says "delete key" but prints field value

**File:** `libbeat/processors/actions/alterFieldProcessor.go` (line 157)
**Current text:** `return fmt.Errorf("could not delete key: %s, %w", v, err)`
**Suggested fix:** keep message intent but pass `valueKey` instead of `v` (or change message to explicitly say value).
**Why:** The message claims key deletion but interpolates the field value (`v`), which is misleading for users troubleshooting processor failures.

## Suggested Actions

- [ ] Update line 162 wording to remove contradictory `successfully` phrase from the failure message.
- [ ] Update line 157 to align placeholder data with message intent (use `valueKey`, or rename the message to `value`).

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> - [x] expires on Sep 2, 2026, 1:32 PM UTC

Start in libbeat/processors/actions/alterFieldProcessor.go at the error paths around lines 157 and 162. Align the delete-key value with the message and remove the contradictory wording, then verify both user-facing errors accurately describe the failures.

Written by the indexing model from the issue text.