refactor: simplify SesState::reset and prefer vec![] in bedrock

[vieiralucas]

Summary

Two small idiom-consistency fixes:

• SesState::reset open-coded "reinit every field except account_id and region" as a 25-line mirror of SesState::new. Delegates by swapping out account_id / region with mem::take and reassigning *self = Self::new(...). The reset path can no longer silently drift from new when new state fields are added.
• bedrock::list_foundation_models used an explicit let mut model_summaries: Vec<Value> = Vec::new();. Replaced with vec![] for consistency with the rest of the crate.

Scope note: several other audit-flagged "inconsistent idiom" sites were inspected and intentionally left alone — they turn out to be necessary (lock-scope clones, async-move captures, Option<String> vs String-with-default semantic distinctions) rather than real inconsistency.

Test plan

• cargo build --workspace
• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --exclude fakecloud-e2e --exclude fakecloud-conformance — 1102 passed, 0 failed
• CI green

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Summary by cubic

Simplifies fakecloud-ses SesState::reset by reusing new, avoiding field-by-field resets and preventing drift; keeps account_id and region intact. Also updates fakecloud-bedrock list_foundation_models to use vec![] instead of Vec::new() for consistency.

^{Written for commit a627835. Summary will update on new commits.}

[cubic-dev-ai]

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