fix: incorrect inject field message for exact class extra field

[tomlau10]

I found this display bug while discussing in an issue: #3211 (comment)

before

---@class (exact) A
---@field a integer
local A = {
    b = 1,  --> Fields cannot be injected into the reference of `A` for `A`.
}

=> the msg incorrectly uses class name `A` for `A`

after

---@class (exact) A
---@field a integer
local A = {
    b = 1,  --> Fields cannot be injected into the reference of `A` for `b`.
}

=> the correct msg should be using the field name `A` for `b`

[Copilot]

Pull Request Overview

This PR corrects the diagnostic message for injecting an extra field into an exact-class table so that it references the field name rather than the class name.

• Update the message payload in inject-field.lua to use the actual field identifier
• Add a changelog entry under Unreleased to document the fix

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
script/core/diagnostics/inject-field.lua	Use field instead of src when building the diagnostic message
changelog.md	Add a FIX entry for the incorrect inject-field message

Comments suppressed due to low confidence (1)

script/core/diagnostics/inject-field.lua:137

• Consider adding or updating a unit test for the exact-class extra-field diagnostic to verify that the message now correctly mentions the field name instead of the class name.

                field = guide.getKeyName(field),