feat(pdf): prefer a configured remote parser in auto, add hosted MinerU support

[q2000s]

Summary

Two changes to PDF parsing, both aimed at making external parsers useful without making
them mandatory:

1. auto prefers the configured remote parser (it used to try local first).
2. mineru_token adds the hosted MinerU API, including the protocol that service
   actually speaks.

1. auto prefers the remote parser

strategy	Remote parser used?	On remote failure	mineru_endpoint required
local	no	n/a	no
mineru	always	the import fails	yes
auto	yes, if one is configured	falls back to the local parser	optional

Behavior change. auto previously tried the local parser first and only used MinerU
when local parsing failed. Since pdfplumber handles essentially every document, the
remote branch was almost never reached -- configuring mineru_endpoint had close to no
effect under auto. Set strategy: local to keep the old behaviour, or strategy: mineru
to require the remote path.

The fallback is per document, not per process.

Provider-shaped dispatch

auto iterates _remote_converters(), which lists the external converters this config
actually enables. "Is a remote parser in play" is PDFConfig.remote_enabled, expressed
over the provider list rather than any single service, and it is what both the dispatch
and the startup preflight read -- so the two cannot drift, and adding a provider does not
mean revisiting either. mineru_enabled remains the MinerU provider's own eligibility
check.

Degradation is bounded, not per-document

A batch that hits a rate limit should not pay for a doomed remote call on every
document, so auto:

• retries 429 and 5xx once after a short backoff -- these usually clear;
• sends 404 and other permanent errors straight to local, since retrying cannot help;
• circuit-breaks a provider after repeated consecutive failures, so the rest of the
  batch skips it; the circuit half-opens after a cooldown, so a service that
  recovers is picked up again rather than being written off for the process lifetime;
• counts degraded documents in pdf_remote_degraded, next to pdf_strategy_effective,
  so a batch that quietly stopped using the remote parser is visible after the fact.

2. Hosted MinerU support (mineru_token)

Config	Protocol	Flow
no mineru_token	self-hosted (v1)	POST {mineru_endpoint}/file_parse, markdown inline
mineru_token set	hosted batch (v4)	POST /file-urls/batch -> PUT -> poll GET /extract-results/batch/{batch_id} -> download full_zip_url

A token is treated as the signal for the hosted API rather than as a credential to attach
to the v1 call, because that service has no /file_parse route at all -- every document
would 404 and fall back. If the batch handshake is refused the parser falls back to
/file_parse once, keeping a token-gated self-hosted deployment working. Endpoints
without a token keep the existing /file_parse contract untouched.

Tests

tests/parse/test_pdf_strategy_dispatch.py, 26 cases:

• auto prefers the remote parser; does not call local on success
• auto falls back to local on a raised error, a timeout, and 404/405/429/500/503
• auto uses local directly when nothing is configured; local never contacts remote
• mineru does not fall back
• token selects the batch protocol; no token keeps /file_parse; a refused batch
  handshake falls back
• 429/500 are retried once, 404 is not
• the circuit opens after repeated failures and half-opens to recover
• batch level: 10 documents with 3 remote failures -- every document produces a
  result, pdf_strategy_effective is correct per document, and the degraded count is
  observable

47 passed across this file plus test_pdf_bookmark_extraction.py; ruff check and
ruff format --check clean.

Verified against the live hosted endpoint: strategy: auto with a token produced
api_mode=v4-batch, a real batch_id, and markdown returned by the service.

Docs

docs/{en,zh}/guides/01-configuration.md: strategy and protocol tables, the
provider-shaped predicate, the retry/circuit rules, and a behavior-change callout.
docs/{en,zh}/about/02-changelog.md: entries for all of the above under Unreleased.

Scope

Self-contained on main; depends on no other PR. tests/parse/ has 123 pre-existing
failures on main unrelated to this change (confirmed against a clean checkout).