feat(connectors): store source state on an HTTP state server

[mmodzelewski]

Source connectors checkpoint through the file state backend, which
writes one file per source to local disk. That ties a source's
cursor to the node it happens to run on, so replacing the node or
failing over to another runtime instance rewinds the source.

Add a second backend selected by state.storage. The HTTP provider
stores each source at {url}/source_{key} on any HTTP-speaking
store, so cursors outlive the process. Concurrency is optimistic:
reads remember the returned ETag, writes are conditional
(If-Match, or If-None-Match: * before the first write), and each
logical save carries a stable Idempotency-Key so a server that
committed a write but lost the response replays that outcome
instead of failing the retry with a spurious 412.

State failures are now classified in the SDK. 5xx, timeouts and
connect failures retry with backoff and end as TransientState, so
the batch is Nacked and the plugin re-polls. Version conflicts and
revoked authorization are PermanentState and latch the provider,
failing later saves without touching the network: another writer
took over, and retrying would only mask the original error. A
classified failure while loading an enabled source's state aborts
startup instead of minting a failed plugin, because treating it as
"no state" would silently rewind the source.

The file backend stays the default and its unclassified load
errors keep the per-connector failure path, so existing configs
parse and behave as before.

[codecov]

Codecov Report

❌ Patch coverage is 89.04236% with 119 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.91%. Comparing base (c0c7493) to head (0274642).

Files with missing lines	Patch %	Lines
core/connectors/runtime/src/state/http.rs	90.76%	53 Missing and 21 partials ⚠️
core/connectors/runtime/src/source.rs	63.88%	12 Missing and 1 partial ⚠️
core/connectors/runtime/src/configs/runtime.rs	89.77%	9 Missing ⚠️
core/connectors/runtime/src/manager/source.rs	33.33%	6 Missing and 2 partials ⚠️
core/connectors/runtime/src/state.rs	89.04%	5 Missing and 3 partials ⚠️
core/connectors/runtime/src/stream.rs	92.30%	0 Missing and 5 partials ⚠️
core/connectors/runtime/src/error.rs	0.00%	1 Missing ⚠️
core/connectors/runtime/src/main.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3940       +/-   ##
=============================================
- Coverage     83.87%   68.91%   -14.96%     
  Complexity     1358     1358               
=============================================
  Files          1212     1213        +1     
  Lines        166843   141845    -24998     
  Branches     134306   109430    -24876     
=============================================
- Hits         139937    97757    -42180     
- Misses        23266    40254    +16988     
- Partials       3640     3834      +194     

Components	Coverage Δ
Rust Core	65.20% <89.04%> (-19.45%)	⬇️
Java SDK	66.67% <ø> (ø)
C# SDK	75.00% <ø> (-1.55%)	⬇️
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	95.87% <ø> (+0.02%)	⬆️
Go SDK	68.32% <ø> (ø)

Files with missing lines	Coverage Δ
core/connectors/runtime/src/api/auth.rs	100.00% <100.00%> (ø)
core/connectors/runtime/src/api/source.rs	74.60% <ø> (-0.14%)	⬇️
core/connectors/runtime/src/context.rs	81.64% <100.00%> (-10.76%)	⬇️
core/connectors/sdk/src/lib.rs	57.14% <ø> (ø)
core/connectors/runtime/src/error.rs	26.66% <0.00%> (-1.91%)	⬇️
core/connectors/runtime/src/main.rs	85.76% <66.66%> (-0.06%)	⬇️
core/connectors/runtime/src/stream.rs	90.09% <92.30%> (+14.75%)	⬆️
core/connectors/runtime/src/manager/source.rs	91.97% <33.33%> (-1.61%)	⬇️
core/connectors/runtime/src/state.rs	86.44% <89.04%> (+0.73%)	⬆️
core/connectors/runtime/src/configs/runtime.rs	77.02% <89.77%> (+12.15%)	⬆️
... and 2 more

... and 381 files with indirect coverage changes

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