feat(be): let an app sign its own session out

[sea-snake]

Design: #4224 (docs/ongoing/revocable-app-sessions.md §8.1 — S14, S15). Overview: #4230.

Motivation

Signing out of an app cleared browser state and invalidated nothing already issued. This is
what a user pressing "log out" should actually trigger.

Changes

app_revoke_session : () -> (). It removes the session record, so the app's access ends
within one delegation lifetime instead of running to the original expiry.

• No authorization check beyond the match refresh already performs, because a caller
  cannot produce another session's principal, so it can only ever remove its own.
• Returns nothing and always succeeds, which makes sign-out idempotent: a client that
  retries, or that signs out twice, gets the same answer without having to reason about
  whether its session was already gone.
• It matches without the expiry check, so signing out of an already-expired session still
  clears the record.

The app deliberately cannot revoke anything else. "Sign out everywhere" is the II
frontend's operation (#4246), not something a dapp can trigger.

Revocation latency is exactly the app-delegation TTL, by construction: revoking stops new
mints, and one already issued runs out. mcp.rs documents the same residue for its grants.

Tests

New session_removal_tests (3): removing one session leaves the others, removing twice
reports nothing removed, and removing the last session keeps the account reference.

PocketIC (3): a refresh that worked before sign-out returns NoMatchingSession after,
signing out three times in a row succeeds every time, and signing one browser out leaves
another browser's session on the same account working.

All 765 canister unit tests pass, plus the sessions suite.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 52b32db.

Security Overview

• 🔎 Scanned files: 9 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/6d3e3b61-710b-4b2b-9c0b-43458b6314b6?scanId=a004c449-8f97-4c9a-ac56-442c0afcc851&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► src/canister_tests/src/api/internet_identity/api_v2.rs     Add app_revoke_session function and update wiring ► src/frontend/src/lib/generated/internet_identity_idl.js     Add app_revoke_session здFunction descriptor ► src/frontend/src/lib/generated/internet_identity_types.d.ts     Declare app_revoke_session method in _SERVICE
Enhancement	► src/internet_identity/internet_identity.did     Extend service interface with app_revoke_session ► src/internet_identity/src/main.rs     Expose app_revoke_session update call ► src/internet_identity/src/sessions.rs     Implement app_revoke_session logic to revoke caller's session
Enhancement	► src/internet_identity/src/storage.rs     Add remove_session functionality and adjust session count management for revocation ► src/internet_identity/src/storage/tests.rs     Add tests for session removal scenarios
Enhancement	► src/internet_identity/tests/integration/sessions.rs     Add integration tests for app_revoke_session behavior

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