Clarify PlayFab e9d1 test endpoint inputs and support title-id overrides

[Daniel-Ric]

Motivation

• Make the PlayFab test endpoint easier to use by documenting expected JSON values and where to obtain the PlayFab XSTS token and entity identifiers.
• Allow PlayFab service helpers to target alternate title ids (used by the e9d1 test flow) without changing global config.

Description

• Add explicit Swagger examples for the /inventory/playfab/test request body and update the README example to show where to get playfabToken and how entityType/entityId are resolved.
• Implement a test endpoint POST /inventory/playfab/test that logs in with an XSTS token, exchanges for an EntityToken, selects title_player_account or master_player_account, and returns inventory for title id e9d1.
• Introduce resolvePlayFabTitleId(titleId) and allow loginWithXbox, getEntityToken, and getPlayFabInventory to accept an optional titleId override and use the resolver when composing PlayFab URLs.
• Add unit tests tests/playfab.test.js which cover the resolvePlayFabTitleId behavior.

Testing

• No automated test suite was run for this PR (documentation-focused commit), and the added unit tests were included but not executed as part of this change.

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Codex Task