fix(tracer): prevent stale ctx.tracing crash on HTTPS keepalive connections

[janiussyafiq]

Description

When apisix.tracing is enabled, the core tracer instruments every request phase — including ssl_client_hello_phase — by allocating a tracing table from lua-tablepool and storing it in ngx.ctx.tracing. On HTTPS keepalive connections, OpenResty reuses the same ngx.ctx object across multiple HTTP requests on the same TLS session.

The bug occurs in the following sequence:

1. ssl_client_hello_phase calls tracer.start(), which allocates ctx.tracing via tablepool and initialises tracing.spans.
2. The first HTTP request completes and tracer.release() is called in the log phase, returning the tracing table to the pool. lua-tablepool zeroes all fields on release — tracing.spans becomes nil — but ctx.tracing is never cleared, leaving a stale non-nil pointer to the zeroed table.
3. On the second HTTP request (same keepalive connection), tracer.start() finds ctx.tracing is non-nil (a zeroed table is still truthy in Lua) and skips re-initialisation.
4. span.new() then crashes at table.insert(tracing.spans, self) because spans is nil.

This fix addresses the root cause at two layers:

• tracer.release(): ctx.tracing = nil is now cleared at the very beginning of the function, before any tablepool operations, so a stale pointer is never left in ngx.ctx for the next request to inherit. A if spans then guard is also added to make release safe when called on an already-partially-cleared table.
• tracer.start(): The initialisation guard is extended from if not tracing then to if not tracing or not tracing.spans then, so tracing is correctly re-initialised even if a stale state is somehow encountered (e.g. diverged HTTP/2 contexts).

Which issue(s) this PR fixes:

Fixes #13200

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change (t/node/tracer.t)
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible