Add CalDAV time-range filtering with server-side query support

[thoscut]

Summary

Implements server-side time-range filtering for CalDAV event queries. The range parameter of CalDavClient::list_events was previously bound as _range and ignored — the start/end parameters of the list_events tool had no effect on any server, contradicting the docs.

list_events now converts ISO 8601 date/time strings to iCalendar UTC basic format and issues an RFC 4791 calendar-query REPORT with a time-range element to fetch only events intersecting the requested window.

Key Changes

• New time_filter module (crates/caldav/src/time_filter.rs):
  • to_ical_utc(): Converts ISO 8601 strings (offset, naive, or date-only) to iCalendar UTC basic format (YYYYMMDDTHHMMSSZ)
  • event_overlaps() / filter_events(): Client-side overlap filtering used by the mock client so tests exercise the range semantics
• LibDavCalDavClient::list_events (crates/caldav/src/client.rs):
  • With a TimeRange, performs a server-side calendar-query REPORT with the time-range filter nested inside comp-filter VCALENDAR > VEVENT (the only placement Nextcloud and other servers honor), then fetches only matching resources via calendar-multiget
  • build_time_range_query() helper constructs the properly-nested RFC 4791 query
  • range = None behavior unchanged (all events returned); public trait signature unchanged
• MockCalDavClient::list_events (crates/caldav/src/tool.rs): applies the same range filtering; new test suite for boundary overlaps, all-day events, missing DTEND, and None range
• crates/agents/src/lib.rs: re-exports the time crate so moltis-caldav uses consistent date/time types without a new direct dependency (no Cargo.toml/Cargo.lock change)
• Docs (docs/src/caldav.md): documents that filtering runs server-side when both start and end are provided

Implementation Details

• Naive date-times (no UTC offset) are treated as UTC; date-only values resolve to midnight UTC
• Events without DTEND are treated as instantaneous, except all-day events which span one full day
• Overlap detection uses inclusive boundaries: events are excluded only if they end before range.start or start after range.end
• Missing or unparseable event times are kept rather than silently dropped
• A unit test asserts the generated REPORT XML nests time-range under VCALENDAR > VEVENT and uses UTC Z timestamps

Validation

Completed

• cargo test -p moltis-caldav — 48/48 passed
• cargo test -p moltis-agents — 418/418 passed
• cargo clippy -p moltis-caldav -p moltis-agents --all-features --all-targets — no warnings
• cargo fmt --all -- --check — clean
• cargo fetch --locked — lockfile in sync
• just build-release — Linux amd64 release build succeeded (target/release/moltis, ELF x86-64)

Remaining

• just lint — full-workspace clippy with --all-features (requires CUDA toolchain; not available in the dev container, covered by CI)
• just test — full-workspace nextest with --all-features (same CUDA constraint)
• E2E verification against a real CalDAV server

Manual QA

1. Configure a CalDAV account in moltis.toml ([caldav] with enabled = true and an account with url/username/password).
2. Call the caldav tool with operation = list_events, a calendar href, and start/end (ISO 8601, e.g. 2026-07-01T00:00:00 → 2026-07-31T23:59:59).
3. Verify only events intersecting that window are returned; events overlapping the boundaries must be included.
4. Repeat without start/end — all events must be returned (unchanged behavior).
5. Optional: capture the HTTP traffic and confirm the REPORT body nests <C:time-range start="...Z" end="...Z"/> inside comp-filter name="VEVENT" within comp-filter name="VCALENDAR".

https://claude.ai/code/session_012UDpqLhTib2oMkPj2roqaB

[thoscut]

Superseded by the upstream PR: moltis-org#1147 (same branch, claude/caldav-time-range-filter-0lrbbu). Closing this fork-internal PR.

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