fix: emit durable_function:true tag on first On-Demand cold start invocation

[jchrostek-dd]

What

Fixes a bug (SVLS-8800) where aws.lambda.enhanced.invocation metric was missing the durable_function:true tag for the first invocation after a cold start in On-Demand Lambda mode.

Root Cause

In On-Demand mode, the Telemetry API delivers PlatformInitStart asynchronously — it arrives at the extension after the Invoke event from next_event(). The extension was emitting the invocation metric immediately on receiving the Invoke event, before on_platform_init_start had a chance to call set_durable_function_tag().

This race was already noted in a code comment (processor.rs:343) for span context purposes, but was not accounted for in the metric tag path.

Fix

Defer invocation metric emission for On-Demand cold starts until PlatformInitStart is received (which sets the durable_function tag). PlatformInitReport serves as a fallback if PlatformInitStart is never received.

SnapStart restores are explicitly excluded from the deferral path — they fire PlatformRestoreStart/PlatformRestoreReport instead of PlatformInitStart/PlatformInitReport, so deferring would cause the metric to be silently dropped.

Changes

• bottlecap/src/lifecycle/invocation/processor.rs: Added platform_init_start_received flag and pending_invocation_metric_timestamp field to Processor. On-Demand cold start defers metric; on_platform_init_start emits it after setting the tag. SnapStart excluded from deferral.

Tests

Unit tests (3 new, 525 total pass):

• test_durable_function_tag_present_on_first_on_demand_cold_start_invocation — simulates the race, asserts durable_function:true present
• test_invocation_metric_emitted_after_platform_init_start_for_regular_runtime — same race, non-durable runtime, no durable_function tag
• test_invocation_metric_emitted_immediately_for_snapstart_restore — asserts SnapStart emits immediately, no deferral

Integration tests (new, all pass against real AWS):

• should emit aws.lambda.enhanced.invocation metric for first cold-start invocation — real Lambda cold start, real Datadog metric
• invocation metric should have a positive count
• should NOT emit a metric tagged durable_function:true for a non-durable runtime

Jira: https://datadoghq.atlassian.net/browse/SVLS-8800