feat: add document input support for text modality

[Kamilbenkirane]

Summary

• Add DocumentArtifact + DocumentMimeType for PDF, DOCX, XLSX, PPTX, CSV, TXT, HTML, MD
• Add Domain.DOCUMENTS with celeste.documents.analyze() namespace (async/sync/streaming)
• Add document= parameter to TextClient.analyze() and all namespace variants
• Wire provider-specific document content blocks for OpenAI, Anthropic, Google, and Mistral

Usage

import celeste
from celeste.artifacts import DocumentArtifact

pdf = DocumentArtifact(path="report.pdf")

# Via documents namespace
output = await celeste.documents.analyze(
    pdf,
    prompt="Summarize this document",
    model="gpt-4o",
)

# Via text client
client = celeste.create_client(modality="text", model="claude-sonnet-4-5")
output = await client.analyze("Summarize", document=pdf)

# Streaming
async for chunk in celeste.documents.stream.analyze(
    pdf, prompt="Extract key findings", model="gemini-2.5-flash"
):
    print(chunk.content, end="")

Provider support

Provider	Format	Content block
OpenAI (Responses API)	base64, URL	input_file
Anthropic	base64, URL	document
Google Gemini	base64, URL (via build_media_part)	inline_data / file_data
Mistral (Chat Completions)	base64, URL	document_url

Architecture

Follows the exact same pattern as image/video/audio input:

• DocumentArtifact(Artifact) with DocumentMimeType — src/celeste/artifacts.py
• DocumentConstraint / DocumentsConstraint — src/celeste/constraints.py
• InputType.DOCUMENT + Domain.DOCUMENTS — src/celeste/core.py
• TextParameter.DOCUMENT on 36 models across 4 providers
• DocumentsNamespace with analyze() (async/sync/streaming) — src/celeste/namespaces/domains.py
• _check_media_support() validates document capability at runtime

Test plan

• 553 unit tests pass (7 new: content block formatting, media support validation, signature checks)
• mypy: 0 errors across 331 source files
• ruff: 0 errors
• Integration tests: 36/36 models pass (Anthropic 8, Google 7, Mistral 7, OpenAI 14)
• All pre-commit hooks pass

Closes #207

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.