test: add unit tests for 5 untested modules

[zeroasterisk]

Summary

Adds comprehensive unit tests for the 5 library modules that had no corresponding test file:

• A2A.FileContent — from_bytes/2, from_uri/2, struct construction, option handling
• A2A.Task.Filter — filtering by context_id, status, status_timestamp_after; pagination with page_size/page_token; history_length truncation; include_artifacts toggle; combined filter scenarios; invalid page token error
• A2A.Task.Status — new/1 and new/2 constructors, all 9 valid states, @enforce_keys validation
• A2A.ID — prefix formatting, 12-char alphanumeric suffix, uniqueness across 100 generations
• A2A.TaskStore — behaviour callback declarations, list_all/2 optional callback, ETS implementation contract check

Motivation

Every lib/ module now has at least one test file. The Task.Filter tests are the most substantial — they cover sorting, each individual filter, pagination edge cases (invalid token, empty token, last page), and combined filters.

Verification

• Tests follow the same patterns as existing tests (ExUnit.Case, async: true, alias conventions)
• No changes to library code — test-only PR

Test plan

• mix test test/a2a/file_content_test.exs passes
• mix test test/a2a/task/filter_test.exs passes
• mix test test/a2a/task/status_test.exs passes
• mix test test/a2a/id_test.exs passes
• mix test test/a2a/task_store_test.exs passes
• Full mix test passes with no regressions

[github-actions]

TCK 1.0-dev Compatibility Results (experimental)

This run is informational — failures do not block CI.

             A2A TCK Compatibility Report              
═══════════════════════════════════════════════════════
SUT: http://localhost:9999
Timestamp: 2026-06-21T23:49:41.921758+00:00

OVERALL COMPATIBILITY: 71.6%

┌─────────────┬────────┬────────┬─────────┬───────┐
│ Level       │ Passed │ Failed │ Skipped │ Total │
├─────────────┼────────┼────────┼─────────┼───────┤
│ MUST        │     44 │     35 │      35 │   114 │
│ SHOULD      │      2 │      9 │       0 │    11 │
│ MAY         │      2 │      2 │       0 │     4 │
└─────────────┴────────┴────────┴─────────┴───────┘

BY TRANSPORT:
  agent_card:    8/10 ⚠
  grpc:          0/72 (72 skipped) ✓
  jsonrpc:       50/100 (30 skipped) ⚠
  http_json:     3/83 (80 skipped) ✓

FAILED REQUIREMENTS:
  ✗ CARD-CACHE-002 (agent_card): Agent Card response should include an ETag header
  ✗ CARD-CACHE-003 (agent_card): Agent Card response may include a Last-Modified header
  ✗ DM-ART-001 (): Response contains no artifacts
  ✗ DM-MSG-001 (): Expected a Message response, but got a Task or no payload
  ✗ JSONRPC-SSE-002 (): Error code mismatch: expected ContentTypeNotSupportedError (-32005), got ParseError (-32700)
  ✗ JSONRPC-ERR-003 (): error.data is absent — A2A errors MUST include ErrorInfo in data array
  ✗ CORE-MULTI-004 (jsonrpc): Expected error code -32001 (TaskNotFoundError), got -32603
  ✗ CORE-HIST-001 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-HIST-002 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-HIST-003 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-HIST-004 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-HIST-005 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-HIST-006 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-GET-001 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_COMPLETED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-CANCEL-001 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-CANCEL-002 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_COMPLETED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-SEND-002 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_COMPLETED' but got 'TASK_STATE_WORKING'")
  ✗ CORE-MULTI-005 (jsonrpc): ('/home/runner/work/a2a-elixir/a2a-elixir/.tck-v1/tests/compatibility/_task_helpers.py', 98, "Skipped: Expected task state 'TASK_STATE_INPUT_REQUIRED' but got 'TASK_STATE_WORKING'")