fail with nice error when trying amend the very first commit

[ianhi]

There was logic for this, but a different, less clear error was being hit first.

# /// script
# requires-python = ">=3.11"
# dependencies = [
#     "icechunk>=2.0.0.dev0",
#     "zarr>=3",
# ]
#
# [tool.uv]
# extra-index-url = [
#     "https://pypi.anaconda.org/scientific-python-nightly-wheels/simple",
# ]
# index-strategy = "unsafe-best-match"
# ///

import zarr
from icechunk import Repository, in_memory_storage

storage = in_memory_storage()
repo = Repository.create(storage, spec_version=2)

session = repo.writable_session("main")
root = zarr.group(session.store)
root.create_array("data", shape=(10,), dtype="i4")

# Try to amend the initial commit
amended_commit = session.amend("Amending initial commit")

would fail with a confusing error:

➜  icechunk-python git:(ian/fix-amend-initial-commit) ✗ uv run tests/test_amend_initial_commit.py
Traceback (most recent call last):
  File "/Users/ian/Documents/dev/icechunk/icechunk-python/tests/test_amend_initial_commit.py", line 35, in <module>
    amended_commit = session.amend("Amending initial commit")
  File "/Users/ian/.cache/uv/environments-v2/test-amend-initial-commit-a826cda45e443de5/lib/python3.14/site-packages/icechunk/session.py", line 377, in amend
    return self._session.amend(message, metadata)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
icechunk.IcechunkError:   × session error: object not found
  │
  │ context:
  │    0: icechunk::storage::object_store::get_object_range
  │            with settings=Settings { concurrency: Some(ConcurrencySettings { max_concurrent_requests_for_object: Some(5), ideal_concurrent_request_size: Some(1) }), retries: Some(RetriesSettings
  │ { max_tries: Some(1), initial_backoff_ms: Some(0), max_backoff_ms: Some(0) }), unsafe_use_conditional_update: None, unsafe_use_conditional_create: None, unsafe_use_metadata: None, storage_class:
  │ None, metadata_storage_class: None, chunks_storage_class: None, minimum_size_for_multipart_upload: None } path="transactions/1CECHNKREP0F1RSTCMT0" range=None
  │              at icechunk/src/storage/object_store.rs:379
  │    1: icechunk::asset_manager::fetch_transaction_log
  │            with transaction_id=1CECHNKREP0F1RSTCMT0
  │              at icechunk/src/asset_manager.rs:447
  │    2: icechunk::session::_commit
  │            with Amending initial commit rewrite_manifests=false commit_method=Amend
  │              at icechunk/src/session.rs:1257
  │    3: icechunk::session::amend
  │            with Amending initial commit
  │              at icechunk/src/session.rs:1134
  │

After this PR the error becoems:

➜  icechunk-python git:(ian/fix-amend-initial-commit) ✗ uv run python tests/test_amend_initial_commit.py
Traceback (most recent call last):
  File "/Users/ian/Documents/dev/icechunk/icechunk-python/tests/test_amend_initial_commit.py", line 26, in <module>
    amended_commit = session.amend("Amending initial commit")
  File "/Users/ian/Documents/dev/icechunk/icechunk-python/python/icechunk/session.py", line 377, in amend
    return self._session.amend(message, metadata)
           ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
icechunk.IcechunkError:   × session error: the first commit in the repository cannot be an amend, create a new commit instead
  │
  │ context:
  │    0: icechunk::session::amend
  │            with Amending initial commit
  │              at icechunk/src/session.rs:1134
  │

Rust coding stuff
I tried with

       let current_snapshot_info =
           self.asset_manager.fetch_snapshot_info(self.snapshot_id()).await?;

instead of what I have here. but then that hits a similar issue where it doesn't work. which is maybe an issue of it's own right?