fix: keep terminal multiline bindings buffer-local

[Silex]

Problem

ai-code-backends-infra--configure-session-buffer used local-set-key, which mutates the buffer's local keymap. Terminal buffers (eat/vterm) share their major-mode keymap as the local map, so binding S-<return> / C-<return> / C-<escape> in one session buffer leaked those bindings into every other buffer of the same mode.

Fix

• Install a buffer-local copy-keymap of the current local map before defining any keys, so bindings stay confined to the session buffer.
• When no multiline sequence is configured, remove stale multiline bindings instead of leaving them around.

Tests

Added an ERT test verifying that configuring one buffer does not leak the multiline bindings into another buffer sharing the same mode map, and that the unconfigured buffer keeps its original bindings.

[tninja]

Thanks for the fix. I reviewed the diff and the keymap change looks good.

The current red unit-tests / ert check is not caused by this PR. I reproduced the same ai-code-test-session-checkpoint-sends-fixed-checkpoint-prompt failure on origin/main, so it is a pre-existing baseline issue unrelated to the multiline keymap changes here. I will fix that CI failure independently.