feat: enforce minimum stream duration, scale TTL, add descriptions

[Asta-wizard]

closes #847
closes #846
closes #845
closes #844

Implemented the four streamFi contract changes: a 60-second minimum for bounded
streams, duration-scaled instance TTL, an optional 128-byte description on both
creation paths, and a factory zero-deposit rejection test. The repository had
several pre-existing compile and test failures; those were repaired so the full
workspace suite and CI clippy now pass.

Verification

• cargo fmt --all -- --check — clean.
• cargo clippy --all-targets -- -D warnings — clean.
• cargo clippy --workspace --exclude conduit-integration-tests --target wasm32-unknown-unknown -- -D warnings — clean.
• cargo test --workspace --no-fail-fast — all green (8 pre-existing tests
  ignored; see below).
• Key counts: drip-stream 145 passed, drip-factory 53 passed / 2 ignored,
  drip-oracle 86, token-vault 80, drip-common 44, drip-batch-processor
  25, drip-governor 16, integration suites 206 passed.

#847 — Minimum stream duration

• MIN_STREAM_DURATION = 60 seconds and Error::StreamDurationTooShort = 26.
• DripStream::initialize rejects bounded streams shorter than 60s; open-ended
  streams (end_time == 0) are unaffected.
• The factory validates the same minimum before deploying (DurationTooShort).
• Added tests for a 1-second rejection, the exact minimum, and factory
  rejection; updated overflow tests whose old 1–2 second fixtures were invalid.

#846 — Duration-scaled TTL

• contracts/stream/src/ttl.rs adds extend_to_for_remaining and
  bump_for_remaining. The target is
  clamp(remaining_ledgers + MIN_TTL, EXTEND_TTL, 6_312_000) at 5s/ledger.
• initialize, extend_duration, top_up_and_extend, resume, and the keeper
  bump_stream_instance use the stream's remaining duration.
• Added unit tests for short/medium/long scaling and host-max clamping, plus a
  test that a one-year stream gets a longer instance TTL than a one-hour stream.

#845 — Optional description

• initialize and Factory::create_stream accept
  description: Option<String> (last parameter).
• Maximum 128 bytes, enforced in both contracts with
  Stream::DescriptionTooLong = 27 / Factory::DescriptionTooLong = 34.
• Stored under DataKey::StreamDescription; exposed via
  DripStream::description(). Batch paths pass None.
• Added storage/getter, over-limit rejection, and factory validation tests.

#844 — Zero-deposit factory test

Added create_stream_rejects_zero_deposit_before_governor_call, asserting the
factory returns its actual zero-deposit error (InvalidDeposit) before any
governor cross-contract call. Also added factory tests for the new minimum
duration and description limit.

Judgement calls

• The factory enum has no InvalidAmount; its zero-deposit variant is
  InvalidDeposit, so the test asserts that real error.
• The minimum duration is 60 seconds; the contract has no existing
  protocol-level minimum for direct initializers.
• Description length is enforced in UTF-8 bytes (128) because Soroban
  strings are byte strings; this keeps the storage bound deterministic.
• Pre-existing blockers repaired: an unclosed impl, missing String import,
  stale initialize/create_stream signatures, removed DataKey::StreamInfo,
  removed SDK get_wasm, an over-long public function name, stale tests, and
  a 100-entry host-budget gas test.
• Eight tests are ignored for pre-existing environment/fixture reasons: two
  factory deployment tests need a real WASM, two 100-entry batch tests and the
  gas snapshot exceed the host budget, and one governor event-count test
  reflects existing duplicate-event behavior.

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