fix: Error at plan when merge/squash commit title/message set with strategy disabled (alt. to #3556)

[ecerulm]

Resolves #3554

Alternative approach. This is one of two proposed fixes for #3554. The other (#3556) suppresses the diff and emits an apply-time warning so the config silently converges. This PR takes the stricter route: it fails the plan with a clear error. Opened as a draft so maintainers can compare the two.

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Background

merge_commit_title / merge_commit_message (and the squash_merge_commit_* equivalents) cannot be set on GitHub while the corresponding merge strategy is disabled. With allow_merge_commit = false, the REST API returns:

HTTP 422 Validation Failed
Sorry, you need to allow the merge commit strategy in order to set the default
merge commit message title and message. (no_merge_strategy)

The current provider avoids the 422 by simply not sending these fields when the strategy is off — but nothing tells the user, so the plan keeps showing a change that the apply silently never makes: perpetual, non-convergent drift.

After the change?

A CustomizeDiff validation fails the plan when the user has explicitly configured merge_commit_* / squash_merge_commit_* while the corresponding allow_merge_commit / allow_squash_merge is false:

Error: invalid github_repository configuration: "merge_commit_title" cannot be
set while allow_merge_commit is false. GitHub does not allow changing these
values while the corresponding merge strategy is disabled; remove them or
enable the strategy

Key details:

• Only explicit config triggers it. "Configured" is detected via the raw config (GetRawConfig), i.e. the value is actually present in the user's .tf. Because merge_commit_* have schema defaults, a config that only sets allow_merge_commit = false and never mentions the title/message is not affected.
• When the strategy is enabled, these fields behave exactly as before.
• The error is at plan time (CustomizeDiff), so it's caught before any apply.

Trade-off vs #3556

	#3556 (suppress + warn)	This PR (error at plan)
Result	Plan converges; ignored values, apply-time warning	Plan fails with an explicit message
Signal timing	Warning at apply (SDKv2 can't warn at plan)	Error at plan
Behaviour change	Non-breaking (previously-drifty config now converges)	Potentially breaking: a config that previously applied (with drift) now fails to plan
Philosophy	Be lenient, inform	Be strict, force the user to fix the config

Verification

Verified against a real repository:

• allow_merge_commit=false + merge_commit_title explicitly set → plan errors ✅
• allow_merge_commit=false, title/message not set → plan succeeds ✅ (no false positive)
• allow_merge_commit=true + title set → plan succeeds ✅

Pull request checklist

• Schema migrations have been created if needed
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed

Does this introduce a breaking change?

• Yes — a configuration that sets merge_commit_* / squash_merge_commit_* while the corresponding strategy is disabled will now fail at plan time (previously it applied with perpetual drift).
• No

[github-actions]

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