fix: align state prefixes with State constants

[hemasekhar-p]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: FirestoreSessionService ignores app: / user: state prefixes, so app and user state are never shared across sessions #1645

Problem:
In FirestoreSessionService.appendEvent, state key routing checked for non-canonical string prefixes (app and user) rather than ADK's standard prefix constants State.APP_PREFIX. Consequently:

Mutations destined for shared application state (app:) or user state (user:) fell through into individual session state rather than being routed to their dedicated Firestore collections (app-state and user-state).
Ephemeral keys matching State.TEMP_PREFIX ("temp:") were not skipped and were unintentionally persisted to durable session state.
The deletion sentinel State.REMOVED was treated as a normal value rather than executing FieldValue.delete() in Firestore or session.state().remove(key) in session memory.

Solution:
Updated FirestoreSessionService.appendEvent to route using State.APP_PREFIX and State.USER_PREFIX, stripping the prefix before saving to the respective Firestore document.
Added an explicit check for State.TEMP_PREFIX to ignore ephemeral state during persistence.
Mapped State.REMOVED to FieldValue.delete() for scoped Firestore mutations and invoked session.state().remove(key) for session state.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Manual End-to-End (E2E) Tests:

Validated the issue before and after fix manually in local.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.