fix(collect): gate ADO merge_commit_sha by merge strategy (closes #96)

[maui314159]

Summary

Narrows the ADO merge_commit_sha emission gate so non-joinable SHAs from squash / rebase completions aren't silently persisted into pull_requests.commit_shas (issue #96, follow-up to #94).

ADO's lastMergeCommit.commitId is actually the preview merge ref (refs/pull/N/merge). For noFastForward (true merge) completions it equals the commit that lands on target. For squash and rebase, ADO creates new commit(s) on target whose SHA is not lastMergeCommit.commitId. The previous code wrote those non-joinable SHAs into commit_shas, producing rows that never match against the commits table.

Strategy matrix

status	mergeStrategy	merge_commit_sha
completed	noFastForward or absent	Some(lastMergeCommit.commitId) (joinable)
completed	squash / rebase / rebaseMerge	None (would not join)
active / abandoned / anything else	any	None (existing #94 behavior)

The None outcome maps to commit_shas = "[]" in upsert_pr, which is the same pre-#94 default — no regression for downstream consumers.

Changes

Single file: src/collect/azdo/pr_fetcher.rs (+210 / -16).

• PrRaw now deserializes both top-level mergeStrategy and nested completionOptions.mergeStrategy, preferring the top-level field when both are present.
• From<PrRaw> for AdoPullRequest gates merge_commit_sha emission per the matrix above. Comparison is case-insensitive (mirrors the existing status check style).
• Module-level doc block at the top of the file documents the matrix and the rationale (refs/pull/N/merge semantics).
• Existing pr_raw_drops_merge_sha_for_non_completed_status test fixtures updated to include "mergeStrategy": "noFastForward" so the completed-status sanity branch still emits a SHA after the gate change.
• 7 new fixture tests covering every strategy plus absent and case-insensitive variants.

Out of scope (deferred)

• /pullRequests/{id}/commits fallback to recover joinable SHAs for squash / rebase — issue Verify ADO lastMergeCommit.commitId against merge/squash/rebase completion strategies #96 lists this as optional; None is a safe default and a no-regression default vs pre-fix(collect): persist ADO PR merge commit SHA in commit_shas (closes #92) #94 behavior.
• Real-org sanitized fixture capture — not available; synthetic schema-accurate JSON is used.
• Symmetric GitHub state-gate bug — filed separately as GitHub PR collector writes non-joinable merge_commit_sha for non-merged PRs (no state gate) #101.

Test plan

• cargo test --lib azdo::pr_fetcher — 35 / 35 pass
• cargo test — full suite passes
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean

Closes #96

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Pull Request opened by Augment Code with guidance from the PR author