feat(event-handler): add per-route validation support

[oyiz-michael]

Issue number: closes #6983

Summary

Changes
This PR adds per-route enable_validation control to Event Handler, allowing routes to override the resolver-level validation setting. This enables incremental adoption of OpenAPI validation in large monolithic Lambda functions.

Implementation:

Added enable_validation parameter (bool | None) to the Route class
Updated all route decorators to accept enable_validation
Modified middleware stack builder to check route-level setting before resolver-level setting
Extended support to BedrockAgentResolver and async middleware chains
Route-level setting overrides resolver-level when explicitly set; None inherits from resolver
Fully backwards compatible - existing code works without changes

User experience
Before:

# Validation was all-or-nothing at the resolver level
app = APIGatewayRestResolver(enable_validation=True)

# ALL routes must have valid OpenAPI schemas
# No way to incrementally adopt validation in a large monolithic Lambda
@app.get("/new-endpoint")  # Must be validated
def new_handler(): ...

@app.get("/legacy-endpoint")  # Also must be validated - breaks if not ready!
def legacy_handler(): ...

Users with large monolithic Lambdas couldn't adopt validation incrementally without migrating all endpoints at once, which is time-consuming and risky.

After:

# Enable validation globally
app = APIGatewayRestResolver(enable_validation=True)

# New/critical endpoints use validation (inherit from resolver)
@app.get("/new-endpoint")
def new_handler() -> NewResponse:
    return NewResponse(...)

# Legacy endpoints can opt-out during migration
@app.get("/legacy-endpoint", enable_validation=False)
def legacy_handler():
    return {"status": "ok"}  # No validation required

# Gradually migrate routes over time without breaking existing functionality

This enables teams to:
Enable validation globally on the resolver
Selectively disable it for legacy routes not yet ready
Migrate routes incrementally at their own pace
Avoid breaking changes during gradual adoption

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
3.2% Duplication on New Code

See analysis details on SonarQube Cloud