fix(mount): preserve default TTL for legacy mount requests

[jlon]

Problem

Mount requests coming from legacy clients (or compatibility paths) may omit ttl_ms and ttl_action in MountOptionsProto.

In the previous flow, missing TTL fields were deserialized as None, then converted into MountInfo as ttl_ms=0 and ttl_action=none. This disabled auto_cache and caused read miss paths to fall back to UFS instead of triggering async cache loading.

Fix Intent

Ensure mount TTL behavior is stable and backward-compatible when optional TTL fields are missing, while keeping default handling centralized to avoid drift across modules.

Implementation Details

• Added centralized mount default constants in curvine-common/src/state/mount.rs:
  • DEFAULT_MOUNT_TTL_STR (7d)
  • DEFAULT_MOUNT_TTL_ACTION_STR (delete)
  • DEFAULT_MOUNT_TTL_MS
  • DEFAULT_MOUNT_TTL_ACTION
• Updated MountOptionsBuilder::new() and MountOptions::to_info() to use these constants.
• Added compatibility backfill at protobuf decode boundary in ProtoUtils::mount_options_from_pb():
  • missing ttl_ms => DEFAULT_MOUNT_TTL_MS
  • missing ttl_action => DEFAULT_MOUNT_TTL_ACTION
• Updated CLI mount defaults to reference shared constants instead of hardcoded literals.
• Updated client config defaults (auto_cache_ttl, default_cache_ttl) to reference the same shared mount TTL string constant.

Tests

• Added regression test: curvine-tests/tests/mount_ttl_compat_test.rs
  • Reproduces legacy payload with missing TTL fields
  • Verifies decode-layer backfill and final MountInfo defaults
  • Verifies auto_cache() remains enabled

Verification

• RUSTC_WRAPPER= cargo check -p curvine-cli -p curvine-common
• RUSTC_WRAPPER= cargo test -p curvine-tests --test mount_ttl_compat_test -- --nocapture

[bigbigxu]

If not set, the default value should be 0, meaning the data will not expire.