…oses #5415)

audit: session-mode-deserialize-fallback (Severity: Medium, audit
status: **DISPUTED** in the re-audit 2026-05-18).

The audit doc's framing — `session_mode = "New"` silently collapses
to `Persistent` — contradicts serde semantics. serde's standard
`#[derive(Deserialize)] enum` errors hard on unknown variants when
there's no `#[serde(other)]` arm, and `#[default]` only kicks in
for `#[serde(default)]` container/field initialisation on a missing
key — not as a fallback for unknown variant strings. The bug as
described does not currently exist.

But: the boundary that makes the audit's framing wrong is also the
boundary that a refactor could quietly remove (adding
`#[serde(other)]` to silence "unknown variant" errors looks like a
reasonable forward-compat tweak). If that ever happens, operators
who type `"New"` instead of `"new"` would land on CLAUDE.md's
"concurrent writes to a single persistent session are undefined"
warning.

So this PR is a documentation-and-regression-test patch:

  * Comment block on `SessionMode` records the strict-variant
    contract and points at the tests so a future refactor knows
    what it's about to break.
  * Four new tests in `agent::tests`:
      - `session_mode_deserializes_lowercase_persistent_and_new`
        — happy path.
      - `session_mode_rejects_capitalised_variant_strings` —
        `"New"` / `"PERSISTENT"` must error with "unknown variant".
      - `session_mode_rejects_empty_string_and_typos` — `""`,
        `"presistent"`, `"none"`, `"default"` all hit the same
        path.
      - `optional_session_mode_default_fires_on_missing_key_not_unknown_string`
        — `#[serde(default)] Option<SessionMode>` is the per-cron-job
        / per-trigger override shape. Pins that `default` only fires
        when the key is MISSING; an explicit `"New"` still errors.

cargo clippy -p librefang-types --all-targets clean. 4/4 tests
pass.

Out of scope (audit's secondary recommendation): extend the
existing `debug!(session_mode=resolved, …)` in
`cron_tick.rs:198-206` to the manifest-load path so operators see
the resolved value at boot. That's a separate observability PR —
the load sites span boot.rs, agent_state.rs, agent_control.rs and
need consistent log targets. Bundling it here would expand scope
beyond "pin the boundary the dispute relies on."

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# Conflicts:
#	openapi.json
#	sdk/go/librefang.go
#	sdk/javascript/index.js
#	sdk/python/librefang/librefang_client.py
#	sdk/rust/src/lib.rs

# Conflicts:
#	xtask/baselines/openapi.sha256

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