chore(commitmentopts): MemoryDB probe target verification + page-cap test coverage

[cristim]

Summary

Combined follow-up to PR #54 addressing two issues that both touch internal/commitmentopts/. Bundled into one atomic PR per the issue surface area + bot economy guideline.

#62 — page-cap test coverage via shared helper extraction

The 6 per-service probers (RDS, ElastiCache, OpenSearch, Redshift, MemoryDB, EC2) ran the same paginated-Describe loop with a hard maxPages=5 cap, but only RDS had a page-cap test. A refactor that broke one prober's loop in isolation could regress the cap silently.

Extracted the loop into walkPaginated(ctx, service, fetchPage) where fetchPage is a per-prober closure that handles each AWS API's specific quirks (Marker vs NextToken field, per-item shape conversion, optional client-side filter for OpenSearch and Redshift). Error wrapping (<service>: <err>) moves to the helper too.

Chose helper extraction over mirroring TestRDSProber_PageCap 5 times because the loops were genuinely identical — only per-item conversion differed, which is what the closure now isolates. New TestWalkPaginated_* covers: page-cap, nil-token termination, empty-string-token termination, token threading, error wrapping, item accumulation. Per-prober Probe tests still exercise end-to-end wiring (which token field, filters) so per-prober wiring breaks remain caught. TestRDSProber_PageCap retained as integration smoke.

Closes #62.

#61 — MemoryDB probe target empirical risk

Issue flagged that db.t4g.small may return empty offerings in some regions because MemoryDB RI coverage skews to db.r6g.*. Resolution path was: verify with AWS creds and switch to db.r6g.large if sparse, OR document the risk if no creds.

This automated session had no AWS credentials (sts get-caller-identity returned NoCredentials), so the probe target stays db.t4g.small. Added a code comment in probe.go documenting the empirical risk, the verification command, and the safe fallback so a future engineer with creds can finish the verification without re-discovering the context. A follow-up comment will be posted on issue #61 noting AWS verification is still needed.

Closes #61.

Test plan

• go build ./...
• go vet ./...
• go test ./internal/commitmentopts/... — passes (ok 3.666s)
• go test ./... — full suite passes
• All 6 probers' _Probe tests still pass (validates per-prober wiring, including OpenSearch/Redshift client-side filters and EC2 IncludeMarketplace)
• TestRDSProber_PageCap still passes (integration smoke for Marker-based wiring)
• New TestWalkPaginated_* covers cap + termination + threading + error wrap + accumulation
• AWS verification of MemoryDB db.t4g.small offerings remains open in commitmentopts: confirm MemoryDB probe target db.t4g.small has reserved-node offerings #61 (no creds available to this session)

Out of scope

• Other commitmentopts behaviour changes
• Frontend changes

[cristim]

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📒 Files selected for processing (2)

• internal/commitmentopts/probe.go
• internal/commitmentopts/probe_test.go

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• Create PR with unit tests
• Commit unit tests in branch chore/commitmentopts-probe-followups

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[cristim]

Note for triage clarity — this PR's effect on the two issues it referenced:

Resolves #62. The page-cap concern was "five non-RDS probers lack equivalent coverage; if any of them silently lost the cap on a refactor, no test would catch it". The shared walkPaginated helper here means all six probers now go through one loop with one cap, exercised by TestWalkPaginated_StopsAtPageCap. The issue body's own preferred path ("extract the pagination loop into a shared helper and test it once") is exactly what landed. Closing #62.

Does NOT resolve #61. The MemoryDB probe target is still db.t4g.small. This PR only added a comment on probeTargetMemoryDB documenting the empirical risk and the verification command (aws memorydb describe-reserved-nodes-offerings --region us-east-1 --node-type db.t4g.small). The actual confirm-or-flip-to-db.r6g.large step still requires a human with AWS creds. #61 stays open.