Add Rust session store identity support

[gimenete]

Context

Persisted session history must be scoped to the authoritative GitHub account identity, not only to the local machine or runtime process. This PR provides the Rust SDK portion of a coordinated runtime-and-consumer rollout so hosts can establish that scope before accessing local session history and preserve it when creating or resuming a session.

The identity consists of:

• authority: the canonical HTTPS origin for GitHub.com or a GitHub Enterprise Server instance
• accountId: the positive decimal authoritative GitHub database ID

It intentionally does not use login names, tokens, local UUIDs, or environment variables as identity transport.

Summary

• add public, non-exhaustive SessionStoreIdentity with Rust fields authority and account_id
• support optional sessionStoreIdentity payloads on session create and resume for compatibility with runtimes that do not yet consume the field
• add Client::set_session_store_identity for the server-scoped sessionStore.setIdentity set/clear contract
• serialize the server-scoped setter as { "identity": { "authority": "...", "accountId": "..." } }, or { "identity": null } when clearing on logout
• cover exact camelCase serialization, omission when absent, and setter/clear wire requests

Integration contract

Consumers should call Client::set_session_store_identity(Some(&identity)) after connecting and before local session-store operations such as list, metadata lookup, or resume. On an account switch, set the new identity before any subsequent session-store request; on logout, call Client::set_session_store_identity(None).

The same SessionStoreIdentity can be supplied through SessionConfig::with_session_store_identity and ResumeSessionConfig::with_session_store_identity so create and resume requests carry the account scope explicitly. The identity is create/resume and server-context configuration only; it is not a mutable session update option.

Related runtime and consumer changes are being coordinated in private repositories and are therefore not linked from this public PR.

Validation

• cargo +nightly-2026-04-14 fmt --check
• cargo test --lib session_store_identity
• cargo test --features test-support --test session_test set_session_store_identity
• cargo clippy --all-features --all-targets -- -D warnings
• GitHub Rust test, documentation, Clippy, bundled CLI, and CodeQL checks

[Copilot]

Copilot review overview

🟡 Changes recommended

The new public struct needs non-exhaustive construction to preserve future API compatibility.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Balanced
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	rust/​src/​types.rs — This new public struct can be constructed with a literal, so adding any protocol field later would…

What changed in this PR

Adds Rust SDK support for account-scoped session-store identities.

Changes:

• Adds the public identity type and create/resume configuration.
• Implements the set/clear JSON-RPC API.
• Tests serialization and wire requests.

File	Description
rust/​src/​types.rs	Defines identity and configuration support.
rust/​src/​wire.rs	Serializes identity into create/resume payloads.
rust/​src/​lib.rs	Adds the client identity setter.
rust/​tests/​session_test.rs	Verifies set and clear requests.

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