fix(storage): use correct header in isMigrated

[designcode]

Note

Medium Risk
Changes the logic that decides whether an object is considered migrated, which can affect read routing/behavior for storage objects. Risk is limited in scope but could cause incorrect migration state detection if upstream header values differ from expectations.

Overview
Updates isMigrated to determine migration status by reading X-Tigris-Read-Source from the HeadObject response and treating block_shadow as not migrated, replacing the prior check that inferred migration from the presence of X-Tigris-Served-From.

Adds the new TigrisHeaders.READ_SOURCE enum value to standardize use of this header across the codebase.

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[greptile-apps]

Greptile Summary

This PR switches isMigrated from checking the presence of X-Tigris-Served-From to comparing the value of a new X-Tigris-Read-Source header against 'block_shadow', and adds the corresponding enum entry.

• The header key lookup responseHeaders[TigrisHeaders.READ_SOURCE] uses the mixed-case string 'X-Tigris-Read-Source', but responseHeaders stores keys in lowercase (as snapshot.ts demonstrates with TigrisHeaders.SNAPSHOT_VERSION.toLowerCase()). This causes the lookup to always return undefined, so isMigrated will always return true regardless of actual migration state.

Confidence Score: 2/5

Not safe to merge — the header key lookup is always a miss due to missing .toLowerCase(), making isMigrated return true for all objects.

A P1 bug causes isMigrated to always return true because the mixed-case header key never matches the lowercase keys stored in responseHeaders, defeating the purpose of this fix.

packages/storage/src/lib/object/migrate.ts — the header key lookup needs .toLowerCase().

Important Files Changed

Filename	Overview
packages/storage/src/lib/object/migrate.ts	Switches isMigrated to check the new X-Tigris-Read-Source header, but omits .toLowerCase() on the key, causing the lookup to always miss since responseHeaders uses lowercase keys.
shared/headers.ts	Adds READ_SOURCE = 'X-Tigris-Read-Source' to the TigrisHeaders enum — straightforward and correct.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[github-actions]

🎉 This PR is included in version 3.1.1 🎉

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• npm package (@latest dist-tag)

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