[FEATURE] Support offline custom dependencies for Code Interpreter sessions

**Is your feature request related to a problem? Please describe.**

AgentCore Code Interpreter provides a fixed set of pre-installed libraries. The
SDK's `install_packages()` helper runs a normal `pip install`, which requires
access to PyPI or another package index.

Code Interpreter sessions running in an isolated VPC without NAT cannot use
this mechanism. Enabling public internet access solely to install dependencies
increases cost and weakens the sandbox's network isolation.

Code Interpreter supports mounting S3 Files and EFS through
`filesystemConfigurations`, but there is no first-class way to declare an
offline dependency bundle or initialization step. Applications must currently:

1. Mount or upload a wheelhouse.
2. Execute `pip install --no-index` manually after every session starts.
3. Configure `sys.path` and `PYTHONPATH` separately for `executeCode` and
`executeCommand`.
4. Detect initialization failures themselves before allowing agent code to run.

This creates duplicated bootstrap logic and makes session readiness ambiguous.

**Describe the solution you'd like**

Add first-class support for application-defined, offline dependencies when
creating a Code Interpreter or starting a session.

One possible API shape would be:

```python
client.start(
identifier=interpreter_id,
filesystem_configurations=[dependency_mount],
dependency_configuration={
"python": {
"requirementsPath": "/mnt/dependencies/requirements.txt",
"wheelhousePath": "/mnt/dependencies/wheels",
"offline": True,
}
},
)

Start with the Code Interpreter session-creation path and the SDK's install_packages() helper, then compare how filesystemConfigurations are passed to sessions. Done means applications can declare an offline dependency configuration, sessions initialize it without public package-index access, and initialization failures are surfaced before agent code runs.

Written by the indexing model from the issue text.