refactor(aws): use common.ErrOutcomeUnknown and drop the internal sentinel

[cristim]

Closes #301 (PR 2 of 2; PR 1 is #329).

What

• Pin pkg to 30bf383 (feat(common): export ErrOutcomeUnknown sentinel for lost-response purchases #329) in providers/aws, providers/azure, providers/gcp and ci_cd_sanity_tests (go.mod/go.sum only, tidied).
• ClassifyPurchaseError, the EC2 and Redshift clients (empty-response paths) and their tests use common.ErrOutcomeUnknown. The purchasecfg.ErrOutcomeUnknown var is deleted: one sentinel, no alias. Error text is unchanged.

Verification (unit tests with fake AWS errors, no live AWS)

• GOWORK=off go build -mod=readonly ./... for all four modules; tests for purchasecfg, ec2, redshift pass; golangci-lint on those packages: 0 issues.
• TestClassifyPurchaseError asserts errors.Is(got, common.ErrOutcomeUnknown) true for 500, transport, 200-undeserializable and false for 4xx/throttle cases.
• Mutation: reintroducing a local sentinel in ClassifyPurchaseError makes TestClassifyPurchaseError fail (run, then reverted).

Consumer follow-ups (need pkg and aws pins bumped to this PR's merge)

• cloud-commitments-mcp#49: errors.Is(err, common.ErrOutcomeUnknown) for the audit status.
• platform #778: persist an outcome-unknown flag and relax the redrive refusal for definite failures.
  CLI has no users. Out of scope: pkg/exchange and other services (go#297).

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📥 Commits

Reviewing files that changed from the base of the PR and between 7e217b1 and 9788bd7.

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⛔ Files ignored due to path filters (4)

• ci_cd_sanity_tests/go.sum is excluded by !**/*.sum
• providers/aws/go.sum is excluded by !**/*.sum
• providers/azure/go.sum is excluded by !**/*.sum
• providers/gcp/go.sum is excluded by !**/*.sum

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📒 Files selected for processing (10)

• ci_cd_sanity_tests/go.mod
• providers/aws/go.mod
• providers/aws/internal/purchasecfg/outcome.go
• providers/aws/internal/purchasecfg/outcome_test.go
• providers/aws/services/ec2/client.go
• providers/aws/services/ec2/purchase_retry_test.go
• providers/aws/services/redshift/client.go
• providers/aws/services/redshift/purchase_retry_test.go
• providers/azure/go.mod
• providers/gcp/go.mod

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[cristim]

Gate review (independent, adversarial) of #335. Verdict: CLEAN, not merged yet (BEHIND main again; queue ruling: #334 merges first).

Reviewed SHA b20c420; after one update-branch the head is 35978de (only d9ead42 from main added; PR's own files diff-empty vs b20c420).

• Pin block: all four modules (providers/aws, azure, gcp, ci_cd_sanity_tests) go.mod and go.sum use pkg v0.0.0-20261009181525-30bf383795db (30bf383, PR 1 merge commit); only go.mod/go.sum changed for them.
• No remaining use of the old sentinel: grep for purchasecfg.ErrOutcomeUnknown / local ErrOutcomeUnknown in providers/ is empty; ClassifyPurchaseError, ec2/client.go:190 and redshift/client.go:215 now use common.ErrOutcomeUnknown. Wrap chain unchanged (%w at ec2 :174, redshift :201; ladder/purchase.go wraps with %w). Message text unchanged ("purchase outcome unknown", pinned by pkg/common/types_test.go:553). 4xx/throttle cases still false in TestClassifyPurchaseError.
• Mutation (local sentinel errLocal in ClassifyPurchaseError, real run): TestClassifyPurchaseError FAILS (500 internal error, 503, 500 over MaxAttemptsError, 200 undeserializable, transport error subtests). Reverted; tree clean.
• Local, GOTOOLCHAIN=go1.26.9 GOWORK=off -mod=readonly: build+vet OK for all four modules; tests ok for purchasecfg, ec2, redshift, ladder, recommendations; golangci-lint 0 issues on touched packages.
• CI at 35978de: all checks pass (Unit, Integration, Lint x4, gosec, Trivy, Snyk, CR); mergeState BEHIND.
  Remaining: after feat(insurance): export the supported contract-term list #334 merges, update-branch once more, CLEAN + CI green, re-check pin block, merge with --match-head-commit.

[cristim]

Final gate: after #334 merged, update-branch to 9788bd7. Diff of the PR's files (go.mod/go.sum x4, purchasecfg, ec2, redshift) between 35978de and the new head is empty. Pin block: all four modules on pkg v0.0.0-20261009181525-30bf383795db. CI 15/15 pass, mergeState CLEAN. Review verdict CLEAN (earlier comment: mutation fails TestClassifyPurchaseError, local builds/tests/lint ok). Squash-merged with --match-head-commit under the go merge lock.