fix(modify): quote --set-field values so a colon-space no longer corrupts the file (#687)

[avrabe]

Closes #687.

Bug

rivet modify --set-field key=value used to write the value RAW under a
block-style fields: sub-map — every sibling setter (--set-title,
--set-status, --set-release, --set-description) already routes
through yaml_quote_inline_scalar, but the set_fields loop skipped it.
A value containing : (or any other plain-scalar indicator — #,
leading -/?/[/{, YAML booleans, …) then produced invalid YAML:

      rationale: CURVE-AGILE: Ed25519 preferred, ECDSA-P256 fallback

The second : reads as a mapping value → parse error on the whole
source file → rivet WARN-and-skips the file → every artifact in it
silently vanishes from the loaded store. Same data-loss shape as
#573/#613/#618 (invalid YAML from a mutation drops the file), just via
the one setter that never got the quoting treatment.

Fix

Route each set_fields value through the existing yaml_quote_inline_scalar
helper before formatting — the fix the issue asked for, one line. Applied at
all three block-style write sites (replace-existing sub-key, insert new sub-key,
create new fields: section). The flow-style branch already went through
serde_yaml, which quotes correctly — left alone.

Acceptance criteria (from the issue's "Suggested fix")

• Route set_field's value through yaml_quote_inline_scalar.
  Applied at all three block-style write sites in
  rivet-core/src/yaml_edit.rs.
• (Deferred, not implemented here) Fail loud on parse error during
  validate/get/list. The issue explicitly framed this as "Separately,
  consider" — decoupling it keeps this PR to a single-file bug fix; the
  silent-skip behavior is orthogonal and warrants its own design/PR (it
  affects the error UX of several commands, not just the mutation path).

Verification

• Three new regression tests (all pass):
  • set_field_quotes_value_with_colon_space_and_preserves_siblings —
    insert branch (existing fields:, new sub-key), sibling artifact survives.
  • set_field_creating_fields_section_quotes_hostile_value — create-fields
    branch (no pre-existing fields:).
  • set_field_updating_existing_key_quotes_hostile_value — replace branch
    (existing sub-key overwritten).
• All 38 yaml_edit tests pass; nothing else in the module was touched.
• End-to-end reproduction of the exact issue command:

  $ rivet modify DD-1 --set-field 'rationale=CURVE-AGILE: Ed25519 preferred, ECDSA-P256 fallback'
  modified DD-1
  $ rivet get DD-1     # succeeds; rationale round-trips verbatim
  $ rivet get DD-2     # sibling artifact survives (used to vanish)
  

  File now contains:

        rationale: "CURVE-AGILE: Ed25519 preferred, ECDSA-P256 fallback"

• CI gates green locally: cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings, rivet validate (PASS).

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✅ All modified and coverable lines are covered by tests.

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[github-actions]

📐 Rivet artifact delta

No artifact changes in this PR. Code-only changes (renderer, CLI wiring, tests) don't touch the artifact graph.