fix(routing): compose routing behaviors before prepare

[Sam Schillace (ramparte)]

Problem

The routing hook was being appended to the final bundle_config after prepare(), but its module source was not available during prepare. That produced a "source-less hook appended after prepare" failure: the hook entry existed in bundle_config but its code could not be imported/initialized because compose/load happened too late.

What I changed

• Compose routing behaviors before calling prepare(): resolve routing config early and include its behavior URI in compose_behaviors so the routing bundle and its modules are available during prepare().
• Add required_behaviors parameter to load_and_prepare_bundle(): callers can mark a subset of composed behaviors as required; failures to load/compose required behaviors now propagate as exceptions and abort prepare. Other behaviors remain optional and still only warn.

Why

• Pre-prepare composition ensures modules/hook sources are present when the bundle's prepare step mounts/inspects hook modules; otherwise a hook entry may be appended to the prepared mount_plan without any corresponding source available.
• Routing is an app-level policy whose hook must be present during prepare; treat it as a required behavior so routing composition failures fail fast rather than silently becoming a non-functional hook in the prepared bundle.

Behavior and failure semantics

• When routing_config is active, its behavior URI is added to compose_behaviors and included in required_behaviors.
• If a required behavior fails to load or compose, load_and_prepare_bundle raises the original error and aborts preparation. This avoids the prepared bundle containing hook entries with missing sources.
• Optional behaviors (notifications, etc.) retain the previous semantics: their composition/load failures remain warnings and do not abort prepare.

Tests

• New unit tests assert that a required behavior load failure propagates (test_required_behavior_load_failure_propagates) while an optional behavior failure stays ignored (test_optional_behavior_load_failure_is_still_ignored).
• Integration-level tests assert routing behaviors are composed when routing_config is present, omitted when absent, and that prepare errors propagate when routing is active.

Compatibility risk

• Low. This tightens semantics for routing-only: systems where routing composition fails will now fail fast. Other behavior semantics are unchanged. If callers depended on routing silently degrading to a no-op, they must handle the new failure mode.

Notes

• The branch was created from feat/cli-session-matrix. This PR is targeted at that feature branch (base: feat/cli-session-matrix) so the diff contains only the fix commit(s) on top of the matrix work.
• Acceptance tests were independently run: 38 passed.

PR created from ramparte:fix/compose-routing-before-prepare -> microsoft:feat/cli-session-matrix