fix(cli): resume selected session in cn ls instead of the latest

[Purv-Kabaria]

Description

Fixes #12927.

cn ls lets you pick a past session from the TUI, but resuming always reopened the most recent session regardless of which one you selected.

Root cause is in extensions/cli/src/session.ts. loadSession() sorts the session files by mtime and unconditionally loads files[0]:

// Load the most recent session
const session: Session = JSON.parse(fs.readFileSync(files[0].path, "utf8"));

The cn ls flow (commands/ls.ts) already communicates the chosen session by setting CONTINUE_CLI_TEST_SESSION_ID — the same env var SessionManager.getCurrentSession() uses to pin the current session id — before calling chat({ resume: true }). But loadSession() never read that var, so the selection was silently dropped and --resume always fell back to the latest session.

This PR makes loadSession() honor CONTINUE_CLI_TEST_SESSION_ID first (loading that session by id via the existing loadSessionById()), falling back to the most-recent session when the var is absent or the id can't be loaded. Plain cn --resume is unchanged.

The fix is intentionally minimal — one guard at the top of loadSession(), no signature or caller changes.

Note: the env var name (CONTINUE_CLI_TEST_SESSION_ID) reads as test-only, but it is already used on the production cn ls path today. I kept the existing name to stay minimal; happy to rename it in a follow-up if maintainers prefer a non-TEST name for this now-user-facing mechanism.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (no user-facing docs affected)
• The relevant tests, if any, have been updated or created

Tests

Added a unit test in extensions/cli/src/session.test.ts under loadSession:

• should load the selected session when CONTINUE_CLI_TEST_SESSION_ID is set — sets the env var and asserts loadSession() resolves the session by id (historyManager.load called with the selected id), does not fall back to scanning the directory by mtime (readdirSync not called), and sets it as the current session.

I verified the test fails on main (without the fix) and passes with it. Existing loadSession tests (including "should load the most recent session" for the no-selection path) still pass — full session.test.ts is green (32/32).

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[Purv-Kabaria]

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